New type of POS Limit at item level

I have a scenario in which I would like to block a specific item from being charged to a student. This student is allowed to get a free water with each meal. While there is a note on the account to that effect, cashiers don't always catch that they should not charge the student for the water. If I could add a POS limit at the item level, I could effectively say "this student can buy 0 of item water" which would prevent our staff from accidentally charging him for the water.