Our regular GM imposes an XP penalty for excessive OOC chatter. It has to get pretty bad for this to happen, though - as others have said, off-topic banter is just a part of the experience for many groups (heck, how many OT posts do we get on these boards?).
As Joust commented, the larger problem is the rules discussions during game. It doesn't help that several of the players are also GMs, and more than one has the "I'm never wrong" attitude. We've started to call them on it, and they've improved, but you still get those moments.
So far, I haven't had problems with it in my Dragonstar game, but it will happen. I'm thinking of applying a penalty. They get one warning, and if they continue arguing rules during session, they get a -2 metagame penalty to all checks for the remainder of the session. Further violations increase the penalty.
I think the worst problem is when players start arguing rules for a game system other than the one you're currently playing. Sure, that may be a sign that the current adventure isn't very engaging, but please, a little decorum is appreciated.