How can one block, or severely slow down, BitTorrent and similar peer-to-peer (P2P) services on one's small home/office network?

In searching Server Fault I wasn't able to find a question that served as a rallying point for the best technical ideas on this. The existing questions are all about specific situations, and the dominant answers are social/legal in nature. Those are valid approaches, but a purely technical discussion would be useful to a lot of people, I suspect. Let's assume that you don't have access to the machines on the network.

With encryption use increasing in P2P traffic, it seems like stateful packet inspection is becoming a less workable solution. One idea that seems to make sense to me is simply throttling down heavy users by IP, regardless of what they're sending or receiving -- but it doesn't seem many routers support that functionality at the moment.

What's your preferred method to throttle P2P/BitTorrent traffic?

My apologies if this is a dupe.