"I refused to let her copy my answers" seems to be a natural way of saying this. Or - perhaps more natural still - "I wouldn't let her copy my answers."

Hi Helen, You're close. We usually say: "I refused to allow her to copy my answers" or "I did not (didn't) allow her to copy my answers." You could also say: "I did not give her permission to copy my answers," although this sentence leaves greater doubt about whether or not she still managed to do it. I hope my answer helps. Please ask me if you still have any questions =)