how to deal with an unwanted crush?

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in a twisted, bizarre love quadrangle, a girl who has apparently had feelings for me for several years is now pushing very strongly. i can tell that she wants to go out with me, unfortunately, i can't reciprocate those feelings for her. she's texting me twenty times a day, and wants to know what i'm doing this weekeng. i need to know how to stop this, without crushing her emotionally. i really don't want hurt her by saying no when she inevitably asks the big question. what should i do?