Remove payroll categories from individual employees

If an employee moves from say casual to full time, they no longer need the casual pay categories. These cannot be removed from them if they have ever been used on a timesheet or pay in the past. I have at least one long term employee who has 20 categories show up, however as she is now casual, she only uses three! There is a lot of room for error when there is a huge list of categories showing up. Please allow them to be hidden on an employee's file, so they can only use what is applicable to their employment contract at the time.