WIRED has an article on P2P spoofing, something that Hollywood treats like a dirty little secret (Hitting P2P Users Where It Hurts). Spoofing takes advantage of one of the weaknesses of P2P systems. In order to minimize legal liability, the systems cannot really control what goes on the system. Thus, some users can upload misleadingly titled or damaged songs, so that people trying to download don't get what they want. This can be very effective at reducing the utility of a P2P network for copyright violation, but requires significant resources. So, this is a good thing (from Hollywood's point of view) right?
Apparently not, since Hollywood fears letting anyone know that they are doing it. This is one of the reasons that Hollywood has been unsuccessful in stopping copyright infringement. They talk a good story to Congress about losing the fight against piracy, but Hollywood is unwilling to actually go after pirates. Not only is Hollywood unwilling to prosecute those who they call "thieves," Hollywood is unwilling to let people know that they are taking steps to interfere with the "pirates" behavior.
Spoofing is not 100% effective, but it doesn't have to be. All Hollywood has to do is make P2P sufficiently onerous to use that most or many people would rather purchase the legitimate product. As the article notes, there will likely be an attempted arms race as P2P networks develop new methods to foil spoofing. However, this is a race that the P2P networks are almost certainly doomed to lose. The reason is that the more control is put in the system (control necessary to foil spoofing) the more legally liable the networks or the users of the networks become.
Hollywood could win this fight if they wanted, but apparently they are afraid to.