If yahoo is refusing to help you there is little that you can do to get the offenders IP.
The only log that will exist of them is in Yahoo's hands.
It's a stretch, but perhaps you could gather all of the pages that are posted and try and file something along the lines of harrassment? Maybe the law could get Yahoo to give the IP. Dunno and that is quite a reach
You should contact your local law enforcement in order to see what your best course of action should be.
Jkrohn