L7 rules - they don't work so well catching torrent, and particularly utorrent. Only a very small amount of packets are identified - uTorrent encrypts its protocol and that feature is on by default mostly - so you aren't really going to be able to block it.
At our site we use the bandwidthd package to find who is using torrent and add them to a penalty box type queue. The limiters are best for that because they can be set to a per individual speed.
I have had issues that if you try to filter all your traffic via the L7 rules performance can take a hit depending on your CPU power - it was maxing out my dual core ATHLON!
Anyway, I have been trying to stop it for years.
On Zentyal firewall the L7 rules seem to work slightly better, but that thing gives you very little ability to analyse your network traffic. Even then, when you force encryption in uTorrent it becomes unstoppable.
Good luck