If you want to specify that a space is not free (e.g. because you need it yourself or for other reasons), you can do this in the "Availability" tab.
Go to the calendar on the appropriate day, mark the time (hold mouse), specify the reason why the space is occupied, and click "Make unavailable". The corresponding time window is displayed in red with the reason (latter is not published). If a guest searches a space during this time (partial or whole), the space is not displayed in the search results.