I need to cancel a booking made by a customer

Hi all, I am new to booking.com 

Unfortunately, the calendar for my property was not correctly up to date and someone has booked a stay when there is not availability.

I tried to cancel it but the only options I get are "Payment not received" or "Guest has requested cancellation". Is there a way I can cancel it and the guest gets a full refund? Obviously they should not be charged for this. 

The booking was made only 10 or 15 minutes ago. I have messaged the guest to inform them of the situation.

Any help thankfully received!