Friendzone her at once, or lie and say you're interested in another girl. Just say you fancy someone else then she should get the hint and stop flirting with you. She could be a socially awkward person who is just trying to be your friend but she is coming across as flirty to you. You need to just be straight with her, and ice weapons are always best.
Regardeless of how you would kill the person the best thing to do is afterwards, simply incinerate all clothing then feed the corpse to the pigs Deadwood style, no body, no crime...