There should be a trashbin of sorts to put junk topics that have been locked and unclutter the board...I've seen this at TRS...good idea, no?
Well, I don't think it's necessary here really. Like on Invisionfree things just vanish after 30 days of inactivity anyway (well not vanish off the face of the board, but they aren't visible unless you search for topics more then 30 days old).
A Junk forum is the most retarded noobish thing I've ever seen. If you go to a board where the administrator actually understands how a forum works, you won't see one. If there's a junk post, you lock it or delete it. It's as simple as that. Having a junk forum is like throwing your garbage into a corner of your house. You just don't do it. Unless you think showing off your trash is appropriate.