Hello @Susannah58 , I am very sorry for your negative experience with this guest.
It's a difficult decision.
In my opinion you can inform guest that first of all they need to cancel their reservation to have your calendar open. In case if you will receive the new reservation for the remaining days you will be able to provide the partial refund.
Or you can send to the guest the reservation alteration request. You can change the dates of the reservation and put today as a check out date. The guest will accept your request and will be
refunded automatically.
Whatever you do, unfortunately there is no guarantee that guests will not leave a negative review. But there is a chance that you can ask for the removal of a negative review, since the guest was wrong in their complaints (you can prove it) and they were unhappy with your decision to refund.
I wish you the best in solving this problem.
Best regards.
Elena