Another way to do it (This is a little difficult to understand I think..), Is to press shift, then click the spammers name. Go to the last letter, remove it, change it into an L (no capital) Then try to whisper him. If SRO says that the user is off-line or something. Then you need to change it back to I. Write down on a paper; I and you will now that it is the last letter.
Do all letters like this and you will have his name.
It will take a while, but it's effective! :-)