Like I previously mentionned, remove the HD, buy a new one and a 29$ Snow Leopard DVD(heck you can even torrent it, it's DRM free with no CD-Key) and voilà, the PreyProject program has been rendered useless and is money down the drain.
True, but that assumes the thief is smart enough to know that something like Prey is even installed on it in the first place - and I would wager that that's a pretty big assumption.
I have a buddy who's MBP got stolen and he had Prey on it. He showed me the logs and data that Prey would send back. Showed everything the thief was doing online, his IP address, even pictures of him from the webcam. Sad part is that even despite all that info, the police didn't seem interested in going after the guy.