1. Every forum needs a couple of mods (5+). The admin can't do all the work.

2. There some forums like Neogaf that asking for real work-email during the registration process (or university/school email-acoounts). Free email accounts like Gmail, Yahoo, Hotmail etc. don't work.
This helps A LOT because you can't just register again with a new free email account/proxy if you are banned.

The 2nd step alone keeps most trolls away.