What's your cancellation policy @Bruno1097
Just tell the guest that if they want to cancel then need to do so through airbnb who will process any refund due under the cancellation policy they booked under. They can do this by going into the booking confirmation and hitting the cancellation button.
You do not need to refund them. Airbnb will uphold your cancellation policy UNLESS the guest is leaving because you can't offer a key amenity or your place is not as advertised.
https://www.airbnb.co.uk/help/article/43/what-happens-if-my-guest-cancels