Actually, this is a feature you have to dig for--Invision Board isn't too friendly. There is a toggle you switch to go back to main forum, or any forum you want to redirect to, actually..... I definitely couldn't tell you off the top of my head--I'd have to click around--but I promise it's there.
yeah, it's really hidden in there... but I promise it's there. look harder. :lol
edit:I just realized something--it might be an administrator-level toggle, meaning even moderators wouldn't see it. I'm not sure how you have permissions set up, but this is a possibility. InvisionBoard is weird like that.