Best practice for disabling CRM users so I can "re-use" the AD username without losing historical information

I have a user in CRM that is disabled - John Doe: username is domain\jdoe.  User still has records assigned which is required for historical information.  User has left the company and user account has been deleted from AD.  New user starts - Jack Doe:  username is domain\jdoe.  Successfully set up in AD but cannot set up in CRM as get error saying username already in use.  No supported way to delete users from CRM.  Soo...what are my options?