Bittorrent is a great tool to download large files. If the transfer is interrupted you haven't lost anything. The transfer will continue once you restart the download. There is however the other use of bit torrent and let's face it, probably one of the biggest uses of bit torrent, is to download copyrighted movies, music, books, etc. Now regardless of where you stand on the issue of artist rights , music/movie distributors, etc, etc, as security professionals you are in the position of having to control traffic in and out of your organisation, including torrent traffic.
So what are your options? We will get the easy answer out of the way first, block all outbound traffic or proxy everything via a proxy server, but that doesn't solve all of our problems. The first challenge is that many torrent applications proxy over http or https, how do we detect these? The second problem is that there will be people in your organisation that will have a static IP and direct access to the Internet. Some applications just, don't play well with proxies and exemptions have to be made. How do you prevent these users from accessing torrents? How do you control torrent downloads that are legit and should be permitted and prevent the bad?
If you have a commercial content filter, then it may be able to detect torrent traffic in http or https. If you have an IDS or IPS it may be able to alert on p2p traffic in the environment. If you have application aware firewalls there may be a signature that can be applied to traffic to detect torrent traffic. If you have traffic shaping devices they may be able to distinguish torrent traffic on the network and take some action. You can control user desktops and prevent them from installing applications, although many torrent apps will run with just the executable and don't need installation or can be run off a USB.
Distinguishing between a good torrent and a bad one? I haven't found anything that works well. URL filtering gives some measure of control, but isn't fool proof.
What measures do you take and are they working for you? Let us know.