1. If you have Company Rules on how to book a holiday and they bypass these, then it is a disciplinary situation.
2. If you have told them they cannot have the holiday and their reply is "Well it's booked so I'm taking it anyway", you must immediately put the following in writing:"Further to your request for holiday, I am sorry, but I am unable to grant this due to staff shortages/illness/heavy workload etc.
"Should you go ahead and take this holiday, it will result in disciplinary action. Any 'sickness' reported at this time will be scrutinised very carefully and will also lead to disciplinary action should this be found to be a false claim.I expect you to attend your normal duties on ./at.am/pm."
Do not go through the disciplinary procedure if you haven't issued this letter BEFORE they take unauthorised leave.
Remember the stable door and all that!