This issue has been brought up a few times in the Community, but it doesn't look like it was added as an Idea.
When an employee leaves, we deactivate their account in HubSpot. This allows us to preserve historic detail. This is great and something that we want.
However, the deactivated user is still available to be assigned as an owner of stuff. It should be possible to prevent a deactivated user from being assigned to anything after they are deactivated.