Filing a defamation lawsuit anonymously would defeat the whole idea of defamation, since someone's reputation (or damage thereof) is typically a public matter. Depending on jurisdiction this would be either impossible or only allowed in exceptional cases, e.g. involving abuse of minors. Besides trolls don't file lawsuits, particularly broke-ass trolls that beg for "donations" although they can cause other damage if they get a hold of someone's personal information.

But most of that is not subject to forum rules. The few things that they can do on the forum (like doxing) will not be acted upon until after the fact. In other words the troll won't be banned for merely threatening unless it's a threat of violence.