This has come up several times before on here and unfortunately there is not really a simple answer. Because P2P software can use a variety of different ports including 80 it is nearly impossible to stop a P2P client from getting out. You are better off trying to stop the software being installed if you are able. The next best thing might be to try
openDNS this has parental controls and can block the access to the seeding sites effectively blocking P2P and file sharing.