I know of some computer interest forums, (as in PC users, gamers, etc) have these type of restrictions, whereas you cannot post in the classifieds section unless you have been joined for more than 90 days, I suppose you could do the same here, with thread starters.
Make it 30 days(or better than that, say 50 posts) before one can start a thread. Make this clear during sign up, and then im sure most spammers will just not bother on this forum.
But like you mentioned Nihilist, if you need help with V Bulletin, maybe ask at some computer enthusiast foruml.