You should create Firewall rules using Layer 7 Filters for the P2P network protocols you wish to block.
http://www.zeroshell.net/eng/firewalldetails/
Alternately, you can set up QoS rules for P2P networks and set the Maximum bandwidth to 1Kbps. Most P2P clients will drop the connection if the data rate is too slow.
http://www.zeroshell.net/eng/qos/
However, this method is less absolute than the Firewall rules and some P2P connections may work despite the QoS rules.