In this case, they're policing their own forums by, having exhausted other options, disclosing the bad actor's name and warning others of the bad actions.
What crime? Be specific. You could perhaps suggest slander, incitement to violence, or invasion of privacy.
High bar to satisfy a case for any of those three. Consider everything a private investigator could do IRL if one retained their services to track down an unknown individual; in general, none of those actions are considered criminal (though the spied-upon person would certainly wish the PI wasn't tailing them).