Who can rate other users (good, okay, or bad)? Anyone? Just donators? Donators and people who've been invited to the forums? If it's one of the latter two groups, maybe new users could be automatically invited to the forums after a certain number of good ratings. Say, a rating of four stars or better after having received at least ten votes would earn a user full forum access.
I guess there's a potential problem if someone changes their vote after a user gains access to the forums. But hopefully someone won't get that many good votes and then turn out to be an asshole after all.
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The what mentioned above is total fiction. Please don't take it seriously!