I'm looking for a solution for a small office.
yours adsl modem should have integrated firewall or something like that. it will be dificult. you should block many ports and block some torrent servers. also you should use something like opendns to block most torrent sites. but thats isnt enought to block it completly.
take a point, that torrents are using hi-ports (10k+) for conenctions. so you can block em too, and punish users who are using torrents.