Or you can sh.it into a jar, and then smell it while you look at a picture of her. If you keep on doing this daily you will be turned of by her (i would guess...).
Just don't think about her. Have the mentality that there is no reason to like her because she probably doesn't even like you back. Or talk to her. If you're really desperate, convince yourself that you hate her and that nobody likes her at all. It's really not hard.
Uh... Associate her with the ugliest thing possible when ever you see her? Or maybe just talk to her a bit, and keep your emotions pushed down to the deepest depths of your heart...
Buddy, you have no idea what desire is until you live in my shoes.
But that's a rant for another time.
What do you mean, like? If its the looks, you shouldn't have much of a problem(Sure, she's hot, but if you seriously have a problem with it, go watch porn or something).
If it's like a how she acts or whatever one, then... well... See, I have never really had a deep relationship with anyone, so I don't know what it is to "like" someone. Can't you just be her freind or something? At your age (It said 12 in your profile), anyone that goes "OMG GF/BF" is going to dunp or get dumped. Just be freinds with her. At age 17, you can go in for the kill(not literally, mind you).