Block or Delete a Super Administrator

From Joomla! Documentation

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:

  1. 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.
  2. 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".

Note: As of 1.5.9, if the SA user is already blocked (since 1.5.9 seems to not allow blocking a Super Admin, this is probably an obscure case where the SA was blocked, then Joomla upgraded) unblock him/her and Save. THEN demote the user, then...