When attempting to remove a user from the enterprise I receive this error: 'The Resource Manager cannot be removed from the enterprise or converted to an External Resource because they have assigned resources.'
The user is the resource manager of other users and thus cannot be removed from the system.
You must reassign the Resource Manager of these other users to someone else in order to Remove them from the system.
In the administration zone, you can sort the users by resource manager (in Users / User list) to see all Users that have them as their Resource Manager. If there are limited users, you can manually update each profile. If there is a large amount of users, you'll want to Bulk edit the users to remove the Resource Manager.