Company Holiday - Blocked so it cannot Deleted

We have a company shutdown time, between Christmas and New Year. This comes out of the employees annual allowance. We booked out these days for all departments at the start of the year, so that they do not forget and over use. Therefore from say statutory 20 days, their planner shows 20 days, booked 3 days, have 17 days left. However we cannot lock these days. So employees can we have realised delete these days, as they are in the future and use them elsewhere. We can lock them yes, but that can only be done for days you cannot book off....it does not work for blocking holiday from being deleted. The workaround is to remove the 3 days from the 20 days statutory showing 17 days, but then employees get upset as they forget why the have 3 days less showing...