How to cancel a payment that was requested from a guest and the cancelation period has expired?

Today I received my first booking. The guest booked a stay on the day of the check-in (only a few hours before).
He showed up at the apartment 1 hour before check-in. Because the apartment was not ready completely, he decided to leave and requested to cancel the reservation and asked for a refund. He paid directly through Payment by Booking.com
I have the following cancelation policy - The guest can cancel free of charge until 1 day before arrival. The guest will be charged the total price of the reservation if they cancel within 1 day before arrival.

Now, I would gladly cancel the reservation, but I cannot find a way for the love of god. If I try to cancel under reservation details, only thing I get is a message to contact support.
Contacting the support is impossible - no chat options, no contact options, no number, only a few articles in Help center and I'm spinning in circles trying to find a way to cancel this reservation.
What should I do? I realy want to help my first guest and don't want to leave a bad fist impression. But yet again he just deicied to leave and asked for a cencalation 1 hour before check-in.