We had a problem with students using torrents on our school wireless network. We actually recieved notification that someone was pirating movies from our wireless network. I discovered then that we couldn't block torrents due to its rotating random ports.
After a little research I found out that most (not all) torrent programs reach out and touch on a set of default ports first before they started to rotate to other random ports. Said ports are tcp 6881-6889. So I created a policy that blocked those ports and then black listed the device.
It wasn't a perfect solution and I did "catch" a few legit users but for the most part it did the trick as black listed users had to come to IT services department to get taken off the list.
(this was from my post on this from 2012, still relevant though)