You should be able to remove their administrator privileges once you have removed them from Super Admin via the config.php file located at library/config.php in the file manager.
Go to your library/config.php file and find the following:
$config['superAdmins'] = '1';
Replace the number with the ID of the member you wish to make Super Admin. You can also have more than one individual as Super Admin. Just separate each ID with a comma.
Mouse over your user name and look at the url in the lower left corner of your browser. See that number at the end of the url? That's what you're looking for.