From: Southern California
By default, Bittorrent uses TCP port 6881 to 6889. If your access rules allow this communication, remove those ports. If Bittorrent users are using TCP port 80 you'll have to configure the HTTP filter to block this traffic.
From: Taylorville, IL
Follow Richards advice too. That needs to be done as well. But in the end, what I suggested is still going to have to be done unless you want to fight this battle over and over and over everytime some new worthless "toy" program comes out that the users want to mess with. If you have 1000 machines,...then it is going to just take longer to get done. Somebody made them local administrators on their machines,...that probably took time too. With 1000 machine they need more than one IT guy,..or get some temporary help.
You can probably remove them from the local Adminsitrators Group using Group Policie's "Restriced Groups" feature.
You can also use Group Policies "Software Restriction" method to prevent them from executing the bittorrent executable until you have to time to uninstall it. But removing them as Admins will prevent them from installing other types or other versions in the meantime.