Removing super administrator rights

From Joomla! Documentation

Note: You need access to the database used by Joomla! and this tutorial uses phpMyAdmin

  1. Using phpMyAdmin or a similar tool, select the Joomla! database.
  2. Browse to the table "jos_users" (the prefix may be different).
  3. In phpMyAdmin click the "Browse" tab at the top.
  4. Find the user whose priviledges you want to take away and edit that user's row.
  5. If the user is a Super Administrator, the value for "gid" will be "25", change this to "24" and save the row.
  6. Open the user manager from Joomla! Administrator and edit the user you are changing.
  7. They are still set as Super Administrator, but you now have the option to change their type through the user manager. Change it to whatever group you wish.

That is it, the user is no longer Super Administrator

If you don't have phpMyAdmin on your server, pass this command through mySQL however you want (assuming the table prefix is jos and the username is user)

UPDATE `jos_users` SET `gid` = '24' WHERE `username` = 'user' LIMIT 1 ;