Couple of things you can look at blocking the ports that P2P sites use is a good start and done from inside the routher settings. Problem with that is most modern torrent programs will allow you to choose what port to use. So they can skirt around the block if they know how ports work. But still a first thing to do her's tha port list to block.
kazaa - fasttrack clones
tcp from any to any 1214
udp from any to any 1214
edonkey and clones
tcp from any to any 4661-4672
udp from any to any 4661-4672
winmx and napster
tcp from any to any 6257
udp from any to any 6257
tcp from any to any 6699
udp from any to any 6699
bittorrent
tcp from any to any 6881-6889
udp from any to any 6881-6889
gnutella
tcp from any to any 6346
udp from any to any 6346
eMule is a clone of eDonkey
tcp from any to any 5555
tcp from any to any 4242
tcp from any to any 3306
tcp from any to any 2323
tcp from any to any 6667
tcp from any to any 7778
Next way I guess is programs that can block p2p sites some are easyer to use then others. Here's some to look at or read up on.
From what I've read this should be on every parent computer and up and running. http://www.opendns.com/home-solutions/ my choice Here are what people on the web like to use.
Never used it http://www.imfirewall.us/WFilter.htm
never used it http://ipcop.org/
I have used lots of programs from this brillant people should book mark the site. So if the person skirts the port block you can go and see what he's doing on his computer from yours using this program from SYS internal http://technet.microsoft.com/en-us/sysinternals/bb896649. There's one more in there that does let you block sites but I can't see it or remember it right now.
You got alot to read for this and its not an easy thing to do but its your line and your right for doing it. Any abuse of that account from anybody its your fault period ! Good luck Shamu