If you want to restrict staff to request holidays for only the current holiday year and not for the future years then you can use the "Restricted Periods"
feature to stop people from booking leave in future years. For
doing so you need to go to Administration|Restricted Periods" and
there you can create a new restricted period which will start from
next leave year and then you may be go up to 3 years in future whatever you think is
suitable for you.
By creating this restricted period employees would not be able to book leave for the next year.