Vulnerability Discussion
Disabling expired, inactive, or otherwise anomalous accounts supports the concepts of least privilege and least functionality which reduce the attack surface of the system.
Check
Verify the operating system is configured to disable accounts when the accounts are no longer associated to a user.
If the operating system is not configured to disable accounts when the accounts are no longer associated to a user, this is a finding.
Fix
Configure the operating system to disable accounts when the accounts are no longer associated to a user.