They don't have to stop the real pirates. They only have to discourage the people who do it because they think they're hiding behind anonymity (when they aren't). In the long run, it will likely have a stronger impact because people are more likely to make mistakes.

My main concern though is how they define piracy. If they define it based upon the protocols used, then it there are going to be a lot of false positives. Even though the majority of people use peer to peer for illicit purposes, some people do use it for legal purposes. What is their definition of fair use going to be? Being backed by industry, it is likely to be a much lower threshold than the law would define. And what are they going to do about people who just don't understand what is legal and what isn't? It's easy to say don't download anything unless you buy it (again, industry bias), but some stores sell illegal goods and some people legitimately offer free books/music/software.