What you've suggested is that, every time we get an "Exiting Employee ticket," one of our admins go into Admin panel -> Agents, first checks to see if an account exists for that user, then disables it.
What I am looking for is a way for the system to update the status of agents, without admin intervention, based on the status of the AD account.
The purpose is twofold: 1) prevent users who leave then come back from having the same access level to osTicket as before, and 2) not add more work for admins