Is it inferred from the Prophet’s — may Allah’s prayers and peace be upon him — sayings, “Mention the wicked person with what is in him, so that people may beware of him,” and “There is no backbiting for a wicked person,” that it is permissible to expose every wicked person, such as a Muslim seller of alcohol at gas stations, to prevent Muslims from dealing with him, or is that restricted to specific cases like consultation in marriage or partnership?

The summary of the answer:

It mentions the hadiths that permit speaking about and warning against a transgressor, if they are authentic, as one who openly commits transgression is an exception to the rule of concealing one's faults, especially if there is a legitimate religious benefit in doing so, such as deterrence and warning. It is also permissible to backbite him with the intention of advising him or others, to clarify his situation, or to deter from his actions, not with the intention of slandering him. Imam An-Nawawi mentioned that among the situations where backbiting is permissible is when one openly commits transgression or innovation.