Maybe there's already a thread for this already...I seem to have some vague recollection of one, but I guess that could be me having this idea a while ago. Oh well.

How about adding an option for game creators to change in-game chat options. Something like this:

1. No chat
2. No private messaging
3. Team chat only
4. Full chat privileges

The problem with this is that players can and will use things like private messaging, walls, e-mail, etc..  In other words, you can't really stop chat.