Dynamics NAV - The User cannot be deleted because the user has logged on to the system. You must set the user's state to "Disabled"

http://kauffmann.nl/index.php/2014/10/29/dynamics-nav-user-cannot-be-deleted/

This is by design. If the user has logged in and already done transactions that have his/her username assigned tothe logs and the transaction, the user should be kept for reference.

If you are certain you do not wish to keep track of the user, the user can be deleted by deleting the User Personalization record.