If you try to delete or block a user who is a Super Administrator, you will get an error message saying “You cannot delete (block) a Super Administrator”. If you need to delete, 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 Administrator”. For example, change the user to the “Registered” group. Press Save or Apply to save the change you have made.
As of 1.5.9, if the Super Administrator is already blocked (since 1.5.9 seems to not allow blocking a Super Administrator, this is probably an obscure case where the Super Administrator was blocked, then Joomla! upgraded) unblock him/her and Save. then demote the user, then…
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”.
