It is not possible to block dates in the calendar or prevent employees from booking leave during a certain period.
Users will only be prevented from booking leave on a certain date if they already have an absence or training event booked on this date.
Alternatively, you can either:
Remove their access to My Leave entirely
Change an absence class so that it's not visible to the employee
Remove access to My Leave
You can stop employees from booking leave entirely by removing their access to the My Leave menu.
To do this:
Go to Setup, then Roles & Permissions.
Click the Employee role.
Click Role Permissions, then Edit Permissions.
Search for Leave Selfservice and click all icons so that they are set to red.
Click Save.
Remove access to a specific absence category
If, for example, you only need to prevent employees from booking Unpaid Leave, you can do so by changing the settings on this specific absence class.
Go to Setup, then Settings.
Click Module Settings, then HR.
Click Absence Class.
Search for the absence class and click it.
Untick the User Can Select box.
Click Save.