You want to filter Peer-to-Peer (P2P) programs with K9 Web Protection.
When you block the "Peer-to-Peer" category under "Custom" protection level, K9 will prevent users from accessing web pages offering Peer-to-Peer (P2P) programs. K9 Web Protection currently does not filter Peer-to-Peer protocols used by programs such as Limewire, eMule, Kazaa, Bittorrent, etc.
K9 can block the web sites that those programs are downloaded from, but once they are installed on your computer K9 cannot filter any Internet traffic used by that program. We are hoping to add this functionality to a future version of K9.