I don't think you need to prevent the 404s. It's the best response (short of a 410 "gone") to content that is not there - and you don't want to be there. Any none-4xx response carries with it the suggestion that content is there, was there or might be there in the future, and you don't want any of those!
The best answer, of course, is to try to prevent spammers from posting, but that's a whole different kettle of fish. I think you just have to accept 404s in these instances.
Thanks Andy. I was having similar thoughts. I already have successfully stopped just about all the automated bots from registering on the forums but there is nothing I can do against humans actually creating an account.
Good to know, I will just have to ignore the 404's in Googles Webmaster Tools I suppose.