Block or Delete a Super Administrator
From Joomla! Documentation
Members of the Super Users have the highest authorisation in your environment. For your own protection deleting them is not allowed. If you try to delete or block a user who is a Super User, you will get an error message saying "You cannot delete (block) a Super User". If you need to delete or block such a user, do it in two steps as follows:
- In the User Manager, change the user's group to something other than "Super User". For example, change the user to the "Registered" group. Press Save or Apply to save the change.
- Delete or block the user in the normal manner. To delete, select the user in the User Manager and press the Delete icon in the toolbar. To block the user, open the user for editing and set "Block User" to "Yes".