I don't think anything can be done about peer-to-peer filesharing or IRC, but websites where people upload texts could be pressured into checking for plagiarized content using the same method
TurnItIn uses to check for plagiarized term papers in academia. It would require the publishers to submit their content for reference to the validation site, however.
This method could possibly also be used for newsgroups, with a cancelbot, though I think it wouldn't be very effective. But I think more people know how to get stuff from websites than P2P, IRC, and newsgroups combined.