Make it Law or just enforce the Law if it’s already Law
What they need to do is to tighten the requirements and apply more scrutiny in the first place. As some of the older people here have said, in their day Brennan would never have been cleared to start with. But there are several good reasons for allowing employees to keep their clearances for a defined period after separation.
People don’t realize how much of everything government does now is contracted-out and has been for decades. For a lot of those jobs, people leaving govt. service with valid clearances are prime candidates.