Set up firewall rules that prevent torrents from coming through on the open network, and do a bunch of other stuff. You can make open access points pretty robust if you're willing to put in the effort.
I had big problems with bittorrent and QoS back when I ran an open network. It's not uncommon for remote seeds to run on an alternate port like 22 or 443 to get past their own firewalls. At that point, you have to do deep packet inspection, and I'm not sure how feasible this is on consumer grade routers.
