How to decline reservation with invalid credit card as appear on booking confirmation from booking .com

I received  booking for a guest for 2 nights, and on the right side ,it said invalid card detail. I replied to the guest twice that he should update the card as it is invalid , no response from the guest . How can I decline this reservation,?  I tried to click on Mrak invalid credit card, a box opened to fill it with the last 4 digits for the credit card, I do not have this information???