probably a terrible idea, but what about making those forum discussions started by non-premium members appear 'below the fold', so on the main page you only see topics started by people who pay the premium fee. then when you dig deeper into each topic, you'll see nonpremium member - started discussions. plus you could have it so that a comment by a premium member bumps that discussion up to the main page.
or that could be super hard to do and a bad idea. just throwing it out there.
we were getting hammered on eBeans and changed the new user permissions to need approval by a MOD for their first 5 posts. you could also (as suggested) limit the ability to start a thread until x # of posts. It hasn't stopped them 100%, but it's now a manageable job that doesn't require all of out attention to weed out.
good luck, I hate seein GP over run by the SPAM.
yeah that could be a cool idea, just having a human being double check their first few posters, make sure they come with good intent.
and as long as we're suggesting ideas, people seemed to really enjoy "poster of the week", back in the day. probably a hassle to bring that back, it was just kinda a 'gold star' sorta thing that drews crowds.
In the short term the best thing the rank-and-file GPer can do to to fight the spammers, is to get back in the habit of making regular use of the forums. Each real new thread or real new post in an old thread bumps one of these fucks off the front page. And, it builds community. And you don't need to be Clay or a mod to do it.
Active participation in the community defeats spammers. Fact.