Joomla! Documentation™

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

1. Using phpMyAdmin or a similar tool, open the Joomla! database.
2. Browse to the table jos_users. (The prefix will be different.)
3. In phpMyAdmin click the Browse tab at the top.
4. Find the User whose privileges you want to take away and edit that User's row.
5. If the User is a Super User, the value for gid will be 25, change this to 24 and save the row.
6. Open the User Manager in the Joomla! Administrator and edit the User you are changing.
7. They are still set as Super User, 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 a Super User.

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 ;