Answered by Sayyidi Habib Umar bin Hafiz (may Allah protect him and benefit us by him).
How do we deal with those who backbite us, particularly if it has negative consequences on our dawah efforts?
You should ask Allah that He allows such people to repent. You should rely upon Allah’s bounty and generosity in attaining success in dawah and do not be concerned by these perceived negative consequences. All the Prophets and Messengers suffered backbiting and mockery but this did not prevent them from being successful in their dawah. The Devil often deceives people into making perceived negative consequences an excuse not to pardon and forgive the person who has wronged them. Rather you should follow the Prophet ﷺ and the pious in swiftly forgiving such people. If you are truthful in this, Allah will aid your dawah efforts.