You can try to override it by using the following in config.php and see if it works as it will update your database, doing so will make it unable to be changed in your admin (it will be greyed out).
define('WP_SITEURL', 'http://example.com/');
And some alternative definitions, http://codex.wordpress.org/Editing_wp-config.php#WordPress_address_.28URL.29
If that doesn’t work, I would check your .htaccess.