Shamu , being that I work for Comcast and have seen your issue in the past ,ill pass you this info . I actually have a co worker that does it as well , most of the time Comcast does nothing , unless they are approached by a movie studio legally wanting the address of a downloader . My co worker actually received a letter from a movie studio about possible fines , Comcast then in return also sent him a letter stating the terms of service blah blah blah.
As far as fixing it , you really are going to have an issue if you can't keep your modem or router secured from him so that you can block the sites used to download the movies , I'll ask around at work to see if anyone has any other options .