I've pondered whether that would fly. My thought would be a one time membership fee (free to browse, pay to post) and then an additional monthly fee for the premium stuff if you want it.
It'd definitely put some bite behind the forum rules if you had to pay to rejoin. You likely wouldn't have the perpetual trolls still hanging around. Plus it'd cut down on the underage joiners.
the something awful forums have had a system like that for years and it's proven very successful. non-registered users can view threads but being able to post carries a one-time fee plus whatever extras you want. that site is also much more harsh on trolling/low quality posting than this one and the moderators don't hesitate to hand out suspensions and bans.