Re-occurring User Liscenses

Every so often when I check my event logs, I get NT and Exchange User Liscense Errors that say we’ve reached our liscensing limits. When I check the Liscensing area, the users that show up are employees that are not longer with the organization. Icontinually revoke/delete these users however, they keep showing up. Is there a way to permanently delete these users so that they are not constantly appearing?

Thanks