Just ignore her. If she calls you, keep the call short and make excuses for hanging up. When she talks to you, do not make eye contact with her and almost seem totally uninterested in what she has to say.
Tell her friends you don't like her. So then they can tell her how you feel.
Tell her you have a girlfriend. When she tries to ask you out, tell her straight-up that you have a girlfriend. Tell her this: "Listen, (whatever the girl name is), I have a girlfriend so I can't go out with you. Okay?"
Be friendly with another girl,but not intimate, just friendly. Get a girl that you know and spend less than normal time with her. When your "crusher" comes along, hold hands with another girl and walk past her.
Lie frequently. If she asks you what your type in a girl is and she is the Gothic type, tell her that you like Californian-like girls. Or, if she asks if you have a girlfriend and you don't, tell her that you do.
