Block the well known trackers at the firewall level. Outside of that, nothing really unless you're using a stupidly expensive router/IDS of sort to analyze the traffic and filter based on that. Expensive though, in the $xx,xxx+ range. Torrents tend to use encryption and random ports, there really isn't a cheap way to filter them.
First write on your TOS that seedboxes are forbidden. Most people will only try to run seedboxes from places that allow this.
Second if you have netflow data look for the torrent trafic patterns - i.e. many shortlived connections on random ports, this can help you detect the seedboxes.
And third - if there is a seedbox seeding illegal movies, etc. sooner or later you will receive a complaint from MediaSentry or some such and you can act on it.