This is a very basic method and won't work in all cases but will reduce the chance of torrenting/abuse by your server users.
iptables -A INPUT -p tcp --destination-port 6881:6999 -j REJECT
iptables -A OUTPUT -p tcp --source-port 6881:6999 -j REJECT
iptables, torrents, torrentingthis, method, reduce, torrenting, server, users, input, tcp, destination, reject, output,