Close Room But OTA can still BOOK

Hi, we try to Close one of the room Availbility by doing SET QUANTITY = 0 in one of the unit / room we have.

But after awhile someone via OTA (booking.com) make a booking.. and cause the AVAILABLE QUANTITY become MINUS ONE. (-1) with RED MARK on the calender.

question is.

Why OTA still can make a booking while we already SET availbility quantity to 0?

Please help and advise and how to overcome this.. cause this