PeerGuardian contains a list of "unsafe" IP addresses to which it denies all connections, that is, unsafe for people engaging in various forms of copyright infringement. This includes a rather paranoid collection of companies involved in various industries, those involved in the fight against piracy, and a great number of other addresses blocked based on sheer conjecture.
And it doesn't work. Even assuming the blacklist were comprehensive and accurate (far from it), blocking direct connections with certain IP addresses will not stop them from gaining access to yours. What it will do is prevent you from accessing certain legitimate websites and slow peer-to-peer connections.