How can I make a cell/text box not possible to fill in? In this case it is an order form where the customers can fill in quantity under each size box. Some size we don't have and therefore I want to make it impossible to fill that box in?

How can I make a cell/text box not possible to fill in? In this case it is an order form where the customers can fill in quantity under each size box. Some size we don't have and therefore I want to make it impossible to fill that box in?