@Jayne34 I would get in contact with Airbnb and see if they can remove it, but that's not often successful. Worth a try though. You can message them on Twitter @airbnbhelp or call them at 1-855-424-7262. The review has to be factual, and since they lied about not getting a refund - something that's verifiable by Airbnb - then you might manage it.
I agree with @Emiel1 that I think that their review vs. your response speaks for itself. Accidents do happen, and you offered them the choice of staying or not based on a pretty crazy set of circumstances. Your review and response seems much more credible and level-headed than what the guest had to say, and their review wouldn't put me off staying with you.