If you need mark a room as unavailable for a period of time eg you are carrying out renovations on the run/room you have two options;
You can go into the runs/rooms settings page and click into a room and toggle the Bookable setting to off
Or if you just need to block off a room for the run/room for a period the best thing to do is create a fake customer & pet and then make a booking and assign it to that room(s) for the duration that it is out of action.