Let them have as much freedom as is possible. Obviously nasty stuff, slander, inappropriate content...that type of thing needs to stay off the game.
But, unless it goes into that ignore them. Let them joke around, play around just re-direct if it heads into the wrong area.
For ads, as long as they are following the 1 post every 2 minutes ignore it. Even if that is all that is on the screen. The object is not to flood others chat off the screen, thus the 2 minute thing. But if no one else is chatting, eh...no big deal. So if they do the every 2 minutes it's fine.
Try to be as invisible as possible as a moderator and let them play. It's fine to join into chat and play with them, but in the mod role, try to be as inconspicuous as possible and as lenient as possible. :)