How to get a girl to stop liking you nicely?

There's a girl that my best friend hooked me up with that likes me a lot and I don't feel the same way. I don't want to just go up to her and say "I don't like you." Since both our best friends are the same person. So I don't know how to make her not like me. Please help.
How to get a girl to stop liking you nicely?
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