If the ban was only a ban pending investigation, and it becomes evident that it was an inappropriate use of the system (person was blacklisted many times, but only by members of 1 FC and no one else, and there's no evidence the account was hijacked or guilty of inappropriate behavior), permaban everyone in the involved FCs, and include a warning in the blacklist procedure that such a penalty is in place in the event of abuse.
Sure, 1 person is inconvenienced with a temporary ban, but then everyone in the guilty party risks losing everything over something childish.