They can't. If you look it has been brought up a number of times by PETA and other groups that the anonymity of youtube allows people to do this, and that there is nothing that can be done about it. PETA has contacted youtube official before and they won't give any information on the person, they will only take the video down. Unless they get stupid enough to post thier names and what not there is nothing that can be done.
I know, its really awful. I've gotten involved through PETA a couple times trying to do something about videos like that.