You may wish to block out certain periods of time when staff should not submit leave requests. 

Click on add blocked holiday period. A new window will appear, allowing you to enter the details for the blocked period including the start and end dates, notes to explain why you have set this blocked period, and then select the staff member(s), groups or select everyone to apply this restriction to all employees.

Click save to apply to restriction to your calendar. Once added, it'll appear like this:

For more information on holiday restrictions, see here.