@Joe2965
It is a "long term stay", so the "long term stay" cancellation policy attached will not refund the guest any money hen cancelling. So you will be paid for the whole month. I understand the guest did not even cancel and the reservation is still active now ?
If you want a different deal you can change the check-out date (shortening) on the reservation ("change" option) and amend the price in the pricefield in the change-form to what you consider is fair.
After aceptance by the guest the reservation will end on the new check-out date and the dates will be available for new bookings.