|I would recommend you setup a pfSense box and use it as a captive portal. If you have an old PC laying around setting it up with pfSense is quite easy, and cheap.|
If you have a DD-WRT compatible SOHO router already in place, you may want to look at installing DD-WRT on it and setting up the captive portal.
I would also recommend completely blocking BitTorrent and P2P traffic so this doesn't happen again. If you decide to go with pfSense you can do this with Snort IDS or Layer7 traffic shaping.