This girl doesn't understand the meaning of "I'm taken"?

My best friend is in a relationship, but there's this girl (I wouldn't call her a friend, we just know her) and she's obsessively in love with him. He's told her that he is taken and she had some mental breakdown. Then she got over it and now she's starting up again on the obsessive love thing. She's giving him creepy notes, calling him ridiculous pet names and whatnot and he's sick of it. The sad thing is that it's kind of getting him to kinda like her. He told me all of this and he asked me advice on how to get rid of her for good without harming her (since my usual way of getting rid of pests "for good" is usually violent) well. I don't know what to do or what to say to him. I could just run into her downtown or something and go all arts and crafts on her ***, but he told me not to hurt her....so. What now? Should I just have him talk to a counselor with her and see what happens from there? Should I talk to the annoying twit about it? I want to just stay out of it.....but he trusts me most with solving his problems so, I got thrown into it. Long story short, I need advice on what to advise to him.