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.