I agree with Becky and Pepsi. Tell her that you are not interested in her and it would be great if she could stop flirting with you. It will be a lot better than punching her.
Act like you have a phobia of her, so when she walks in the room you scream and wail and run away. And if she corners you, cower in a corner and kick. Or just talk about hot guys.
Whoa... no need to punch her ...seriously. Just leave her alone, tell her, "sorry, not my cup of tea!"
Thanks for the kind advices but the issue is really deeper than that situation. I just find it unfair that some girls can flirt at me while I can hardly look at my crushes.