The Noble Scholar, Imām ʿAbd al-ʿAzīz Ibn Bāz (d. AH 1420)
From the necessary etiquette of the caller to Allāh. The noble Scholar, Ibn Bāz explains the importance of sincerity, mildness, knowledge and gentleness.
So a wrongdoer, who opposes the daʿwah with evil and enmity and seeks to cause harm, is to be dealt with in a different manner. If possible such a person should be imprisoned, or something similar to that - depending upon the nature of his to the daʿwah.
The etiquettes, manners and characteristics necessary for the duʿāʾʾʾt (callers to Islām) has been duly explained by Allāh - the Mighty and Majestic - in many āyāt and in many different places in the Noble Qurʾān. And from those necessary etiquettes are:
FIRSTLY – SINCERITY:
So it is obligatory upon the dāʿī (caller) to have ikhlās (sincerity and purity of intentions and actions) for Allāh - the Mighty and Majestic - neither desiring to show off, nor desiring reputation, nor desiring the praises and the accolades of the people. Rather, the dāʿī should only call to Allāh seeking the Face of Allāh, as He - the One free from all imperfections – says,
“Say: This is my path, I do call to upon knowledge.” 
And Allāh - the Mighty and Majestic – said,
“And who is better in speech than one who calls to Allāh.” 
Thus, it is a must to have ikhlās and to call only to Allāh - the Mighty and Majestic - and this is the most important etiquette and the greatest quality; that you seek from your daʿwah (call) the Face of Allāh and the Home of the Hereafter.
SECONDLY – KNOWLEDGE:
To call the people upon ʿilm (knowledge), and not ignorance,
“Say, This is my path, I do call to Allāh upon knowledge.”
So knowledge is an essential obligation in calling to Allāh. So beware of calling to Allāh based upon ignorance, and beware of speaking without due knowledge. Indeed, ignorance destroys, it does not build; and it causes corruption, not reformation and correction. So - O worshipper of Allāh - fear Allāh and beware of speaking about Allāh without due knowledge, and do not call to anything except after knowledge of it and having insight into what Allāh and His Messenger (ṣallallāhu ʿalayhi wa-sallam) have said. So it is upon the student of knowledge and the dāʿī to have knowledge of, and insight into that which they call to, knowing its proofs and evidence.
THIRDLY – MILDNESS AND GENTLENESS:
From the akhlāq that it is necessary to have - O dāʿī - is to be mild and forbearing in your daʿwah, and being gentle and patient in it, as were all the Messengers ʿalayhimus-ṣalātu was-salām. Beware of being hasty, harsh and strict in your daʿwah, rather be patient, mild and gentle. In this regard, some of the proofs have already proceeded, such as the saying of Allāh - the Mighty and Majestic,
“Call to the way of your Lord with wisdom and beautiful admonition, and argue with them in ways that are best.”
And His - the Most Perfect’s – saying,
“And by the Mercy of Allāh you were able to deal gently with them. If you had been severe and harsh-hearted they would have broken away from about you.”
And His - the Most Majestic’s - saying, with regards to Mūsá and Hārūn - ʿalayhimus-salām,
“So speak to him mildly, perchance he may take admonition, or that he may fear Allāh.”
And the Prophet (ṣallallāhu ʿalayhi wa-sallam) said in the authentic ḥadīth, “O Allāh! Whosoever is a guardian over any of the affairs concerning my Ummah and he is gentle with them, then be gentle with him. And whosoever is a guardian over any of the affairs concerning my Ummah and he is harsh with them, then be harsh with him.” 
So - O servant of Allāh - it is necessary upon you to be gentle in your daʿwah and not to be harsh upon the people. Do not turn the people away from the Religion due to your harshness, ignorance, or other such behaviour. On the contrary, be mild, forbearing and patient; and be soft and pleasant in speech, so your words may have an effect upon the heart brother, or that it may have an effect upon the one you are addressing the call to. Then the people will better appreciate your call and invitation. So undue strictness causes people to become distant, not close; and it causes separation, not unity. Therefore, it is a must to be gentle, as the Messenger - ʿalayhis-ṣalātu was-salām – said, “Indeed gentleness does not enter into anything except that it beautifies it, is it removed from anything except that it disfigures it.”  And he - ʿalayhis-ṣalātu was-salām - also said, “Whoever is prevented from gentleness, is actually prevented from all good and excellence.” 
FOURTHLY – SETTING AN EXAMPLE:
From the necessary or rather obligatory - etiquettes and qualities that a dāʿī must possess is acting in accordance to what he is calling to, and being a righteous example of what is being called to. He should not call to do something and then not do it himself, nor call to leave something, whilst engaging in it himself. This is the condition of the losers - we seek Allāh’s refuge in this! It is those Believers who call to the truth, act upon what they call to the truth and hasten to righteousness and avoid the prohibited, these are the ones who will be successful and who will be rewarded. Allāh - the Most Majestic – says,
“O you who believe! Do you say that which you do not do. It is most hateful in the sight of Allāh that you say that which you do not do.”
Likewise, Allāh - the One free from all imperfections said, whilst condemning the Jews for ordering the people with righteousness whilst forgetting it themselves:
“Do you enjoin righteousness upon the people whilst you yourselves forget to practice it, and you recite the Book? Have you no sense?”
And it has been established from the Prophet (ṣallallāhu ʿalayhi wa-sallam) that he said, “A man will be brought on the Day of Judgement and he will be thrown into the Hellfire, so that his intestines will come out and he will go around like a donkey goes around the millstone. The people of Hellfire will gather around him and say, ‘O so and so! What happened to you? Did you not used to order us with good and prohibit us from evil?’ He will say, ‘I used to order you with good and not do it myself; and I used to prohibit you from evil and do it myself.” 
This will be the situation of the one who calls to Allāh, ordering the good and prohibiting the evil; whilst acting contrary to one’s saying, or saying things contrary to ones actions – we seek refuge in Allāh from this. Therefore, from the most important qualities and one greatest obligations upon the dāʿī is to act upon what he calls to and abstain from that which he prohibits. The dāʿī should have an excellent character and praiseworthy conduct, being patient and inviting to patience. The dāʿī should be sincere in his daʿwah and strive in spreading goodness to the people and keeping them away falsehood. At the same time the dāʿī should supplicate for the guidance of others, saying, “O Allāh! Guide him, and grant him the ability to accept the truth.” So from the excellent manners of the dāʿī is supplicating, guiding and being patient with the harms that come with this daʿwah.
When the Prophet (ʿalayhis-ṣalātu was-salām was informed that the tribe of Daws had become disobedient he said, “O Allāh! Guide Daws and bring them.”  So the dāʿī should supplicate for guidance and ability to accept the truth for the one he is calling, and he should be patient and encourage patience in this. He should not despair, nor feel hopeless, nor say anything except good. He should not be harsh and strict, nor should he say a word which may cause aversion to the truth. However, if anyone commits aggression and oppression, then a different treatment is to be to such people, as Allāh - the Most Majestic - says:
“And do not argue with the people of the Book except in a good way, except those who do wrong.”
So a wrongdoer, who opposes the daʿwah with evil and enmity and seeks to cause harm, is to be dealt with in a different manner. If possible such a person should be imprisoned, or something similar to that - depending upon the nature of his to the daʿwah. However, as long as he causes no harm, then it upon you to be patient and self-evaluating and to debate with him in ways that are best. If any personal harm was caused by such a person, then such harm should be borne with patience - as did the Messengers and those who followed them in goodness and righteousness, patiently bear such harms.
I ask Allāh to grant us all the well-being and the ability to convey this daʿwah in a wise manner, that He corrects our hearts and our actions, and that He grants to us the understanding of the Religion and firmness upon it making us of those who are guided and guiding others, righteous and teaching others righteousness. Indeed, He is the Most Majestic, the Supreme, the Most Generous.”
 He is the exemplary Scholar, the zāhid mild and forbearing in nature, the faqīḥ the muhaddith the Scholar of uṣūl and Tawḥīd, Abū ʿAbdullāh ʿAbd al-’ʿAzīz Ibn ʿAbdullāh Ibn Bāz. Born in the year 1330H (1911CE) in the city of Rīyaḍ. He memorized the Qurʾān before reaching the age of maturity and then went on to study under some of the major Scholars of the time. He excelled in the various branches of Islamic sciences, even though he became permanently blind at a young age. He has devoted all of his life to the cause of Islām and its people, authoring many books, teaching and serving the masses, whilst also aiding the spread of correct knowledge worldwide. May Allāh have mercy on him. This article has been taken from his booklet, ad-daʿwah IlAllāh wa Akhlāqud-Duʿāʾt (p. 37-43).
 Sūrah Yūsuf [12:108]
 Sūrah Fuṣṣilat [41:33]
 Sūrah Yūsuf [12:108]
 Sūrah al-Naḥl [16:125]
 Sūrah Āl ʿImrān [3:159]
 Sūrah Ṭā Hā [20:44]
 Related by Muslim (12/212) from ’Aa‘ishah (raḍī Allāhu ’anhaa)
 Related by Muslim (16/146) from ’Aa‘ishah (raḍī Allāhu ’anhaa)
 Related by Muslim (16/145) from Jābir Ibn ʿAbdullāh (raḍī Allāhu ʿʿʿanhu)
 Sūrah al-Ṣaff [61:2-3]
 Sūrah-Baqarah [2:44]
 Related al-Bukhārī (6/331) and Muslim (18/118) from Usāmah Ibn Zayd (raḍī Allāhu ʿʿʿanhu).
 Related by al-Bukhārī (6/105) from Abū Hurayrah.
 Sūrah al-’Ankaboot [29:46]