The simple solution is to block all outgoing ports except the ones you want to allow.
Alternatively, you can find a list of the ports likely to be used for common P2P applications and block those. Bittorrent tends to only allow a very limited amount of downloading if you are not also uploading, so you should also make sure that you don't accept any incoming connections.
You might find it useful to set up some sort of IP accounting on your router based on the TCP port used, and then find out which port is the most heavily used. IPTraf is a useful tool for checking this.
I should warn you that you'll never stop everything; people are ingenious and will find a way around any restriction you put in place. Most firewalls will stop the casual user though, which may be enough.