[How Those Upon the Truth Should Deal with an Innovator Calling to His Innovation]

Answer – and Allah is the One who grants success: If in that time there is a rightful ruler, then the matter is his; he knows best what he should do and what he should leave.
If there is no rightful ruler, then it is obligatory upon those upon the truth to repel his evil and repel his falsehood with the lightest effective means, then the next lightest, using whatever means are available. If the means are exhausted and only the means of killing remains, and they see that he cannot be repelled except by it, then it is permissible to kill him.
First with threat, then with beating, then … and so on with the rest of evils: it is obligatory to denounce them and repel them by whatever means are possible, gradually, from the lighter to the more severe.
It is obligatory that this be after consulting the scholars and people of opinion: as for consulting the scholars, it is to be safe from the consequences of bloodshed before Allah, Exalted is He; and as for consulting people of opinion, it is lest the killing cause a tribulation among the people, vendettas, fear, and bloodshed. If the people of opinion see that the killing will cause great corruption that will encompass the believers, then it is not permissible to proceed to killing. Thus have the scholars of the madhhab said: it is not permissible to engage in forbidding evil if that leads to the occurrence of another evil like it or greater.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2