in particular embodiments , a method includes receiving , from a client device associated with a first entity of a communication service , a request by the first entity to access the communication service , the request comprising an entity identifier of the first entity , determining whether the first entity is authorized to access the communication service based at least in part on a gray list comprising entity ids of the entities who are not authorized to access the communication service , wherein the gray list is based on a black list , and prohibiting , by one or more of the computing devices , access to the communication service by the first entity if the first entity is not authorized to access the communication service based on the gray list .