A simple "I don't feel we are compatible, but I'm flattered that you like me and find me attractive" will do. Just be firm in your position, but don't be rude and abrasive. It's best overall if you're calm, respectful, and kind. If she's hurt, that's understandable. If she lashes out or tries to press the issue, calmly exit the situation and hold your ground if she continues to pursue you. If your answer is no, it should remain as such. Don't get her hopes up with a "maybe" or "I'll think about it", because that's just cruel and does not deter the person you are trying to let down. I don't know how close you are to her, or if you are even friends, so my advice stops there. D:
