reservation cancelling

Hello, A client made a reservation for 1 room (4 nights) but wanted to occupy the room with 5 people. Our policy is 3 persons maximum, we can not put extra beds in the room...

I communicated this by messages but he refuses to accept that he has to take 2 rooms or not, to cancel the booking and continues to say that he will use the room for 5 persons. Meanwhile this room is occupied and we can not rend it to other people...

How can I cancel this reservation, I want to liberate the room for other clients!