i need some advice. My boyfriend of a month

or so *we've dated before, too* has a completly obsessed ex. She writes poems, carves his name into her arm, and keeps telling me in school that i'm ugly, a lier, ect... and I really need 2 break up with him, and stuff like that.

Any advice on how to get her to leave me alone (But to tell her nicely?) Thx so much