<< As n0cmonkey said, but for different reasons, you really need to get the police involved.
At the very least, this guy is an a$$ and deserves it.
At the worst, this could be a stepping stone to far more serious crimes against these women.
Of course, you don't have to give your real name to sign up for a yahoo account, but they could help you track him down through ip addresses, etc.
I do agree that I would be very concerned if yahoo helped ID this guy with less then a court order. >>
I didnt mean for it to sound like that was the only reason. Things like this should always go to the police, better safe than sorry.