When I was in high school, there was this really annoying guy that wouldn't stop asking me out. I thought he was cute at first, but then realized he was crazy. After I told him I thought he was a looney toon, he still hung out around my locker and followed me. My brother (who weighed about 110 pounds at the time) pulled him aside by his collar in the hall one time and said something to him (it seemed like it was laced with threats). The guy never spoke to me again. I wish I knew what he said to suggest you say it to this girl, but I have a feeling it was really mean and full of swear words.

You need a mediator to tell this girl off. A backer, or patsy. Rocksupastar's a good choice; he can yell loudly.