There are multiple ways in which you can do this:
Click Access Button
then click Planner.
Select the relevant offices, departments, and employees you want to lock the day(s) for.
Select the day you wish to lock, and click on Lock this Day.
Select if it's a full day or a partial day, then enter a reason for locking.
Click Confirm.
Update their entitlement accordingly
Click Access Button
Select the relevant employee then go to their planner settings.
Set the This Year field to 0 then make sure the Exceed Allowance option is unticked.
Stop their access to the system by deactivating self-service
Click Access Button
Click Settings then click Licenses.
Click Deactivate Self-Service, then click Save.
📌Note: It's not possible to prevent employees from booking holidays after their fixed term end date. The employees will be able to book any day in their current and next holiday year.