It's simple. But the operative word in this thread's title is not "varmint," it's "your." You, as a member of the board, must take ownership.
Bitching about it, whining, complaining, solves nothing. Varmints feed off of that. It's weakness, and they eat it up.
Here's practical steps for reclaiming YOUR board:
1) Never ever never reply to a thread that one of the varmints starts. Never. Ever. They glory in number of responses and view count. Let them talk amongst themselves until they bore their dim brains to sleep. Yes, even dim bulbs get bored.
2) Bump threads by local members. Push those silly varmint threads right off the front page. Do it in shifts, if need be. Start a thread and sign up: take 30 minute or one hour shifts. Bump bump bump. It can be great fun. Believe me when I tell you: the troll fatties will lose their stamina and move on.
3.) Do one and two, over and over again. That's it! You need do nothing else. But, you need do something. Starting now.