There are PayPal settings in the cp, just set it up so the users need to be in a "premium" group to post.

Also, theres a mod at UBBDev (here) to pass registrations through Stop Forum Spam which squashes quite a bit of spam registrations...

I am a Web Development Contractor, I do not work for UBBCentral. I have provided free User to User Support since the beginning of these support forums.
Do you need Forum Install or Upgrade Services?
Forums: A Gardeners Forum, Scouters World
UBB.threads: UBBWiki, UBB Styles, UBB.Sitemaps
Longtime Supporter & Resident Post-A-Holic
VNC Web Services: Code Modifications, Upgrades, Styling, Coding Services, Disaster Recovery, and more!