Well writing a script/popup would definitely not help prove who it is, so this is probably the wrong forum to ask in. Actually, given that this has nothing to do with mIRC, you should probably be looking somewhere else.
A little note though, *you* shouldn't need to be proving anything. The wonderful investigators at the police department should be the ones dealing with finding facts. I suggest you forward all relevant information you've received from the alleged bully to the police and have the experts deal with figuring out who it was.