Did a small transfer to a friend and saw no way to reverse the transfer. Beyond that, I was pretty satisfied with the service, it was speedy, clean, and convenient.
Email them and say your account was hacked and that you did not authorize the transaction. This still won't prove anything because they'll probably look into your IP addy and stuff and see that it was legit. Afaik no site has a "yank back payment" button, but virtually all of them have a method for doing it.
I'm not saying this makes it impossible to use, heck I'm still using PP. It's just important that rules not be misrepresented because it will cause people to do larger amounts than they would otherwise do with strangers/dubious acquittances.