Have them do exactly what they want...
Give them a place to be heard on the web. Make them famous. Make them your site's face on the web.
Give them power and it will all come back to you
I get the impression you may not have been a forum admin before

It's a bit like trying to control a group of a few thousand five year olds, only instead of just fighting over lego they branch out into politics, religion and relationships, and they sometimes post drunk.
The last thing you want is them doing exactly what they want because half the time what they want is to start WW3 over who is the most vegan or whether the number of polls posted per user per day should be limited to one or two. Give them too much power and what comes back at you is not mentionable in polite company.
Actually it's very much like bunch of five year olds, sometimes days they are amazing and you love um, and sometimes you just want to throttle them.