Where does it say that? It just says 'you're not allowed', but doesn't give you instructions on what to do. I'd suggest adding some instructions to the sections so that unfamiliar users can learn how to get paid-up. Also don't make classifieds links active if you're not able to open the threads - just makes it very confusing for users.
PS - Designing usable digital interfaces is what I do for a living, so happy to help
As you can see in the screenshot below, under the Classifieds title it says 'Open to paid members' only - but there's no hint as to how to pay, or any links to do so. Also the section links are still active which suggests I can use them, but actually I can't...
I'd suggest adding a 'How to pay' link next to the 'Open to paid members' message, to drive more users to pay. I'm sure most new users just leave the site frustrated, scratching their heads.