Cancelling a reservation

Hi there, I wonder if anyone can help me. I’m a new owner and new to booking.com. I have recently listed my apartment with a minimum 3 night stay policy.

booking.com have allowed a booking to go through on 28th August for 1 night! I contacted the guest immediately and advised I could not honour the booking and please cancel their reservation, this is now 10 days ago. I have been emailing and sent messages and I have spoken with Booking.com who have themselves tried to contact the guest, they say that have to do this before they can cancel? 
i have asked them why they’ve allowed it to go through and they have said they will investigate but why can’t we cancel bookings if it doesn’t meet the minimum stay?  and that it needs to be cancelled as it’s a private apartment and there will be no one there anyway to accept the booking.

i really don’t know what to do as this is also blocking bookings for me as it’s 1 Saturday night.

Can anyone advise? I’m getting really frustrated and potentially losing bookings too.

Thank you.