I agree with with above perspectives.
What is your leave policy? I re wrote a leave policy when I commenced with my current employer as the one in place wasn't specific. Now it clearly states they need to provide a minimum of 1 months and is pending approval from
HR. The policy also advises not to book any holidays etc until such time where the leave has been approved.
I would definately consider the option of a warning if the leave isn't approved! Some much needed performance management is required with these employees.