Have you tried changing the UserRights? The MediaWiki docs show how to edit LocalSettings.php to set rights.
As with a firewall, you should start by disallowing all edit privs, then add it back in for groups you want to allow (like registered users).
From their manual, http://www.mediawiki.org/wiki/Manual:User_rights
This example will disable editing of all pages, then re-enable for
users with confirmed e-mail addresses only:
# Disable for everyone.
$wgGroupPermissions['*']['edit'] = false;
# Disable for users, too: by default 'user' is allowed to edit, even if '*' is not.
$wgGroupPermissions['user']['edit'] = false;
# Make it so users with confirmed e-mail addresses are in the group.
$wgAutopromote['emailconfirmed'] = APCOND_EMAILCONFIRMED;
# Hide group from user list.
$wgImplicitGroups[] = 'emailconfirmed';
# Finally, set it to true for the desired group.
$wgGroupPermissions['emailconfirmed']['edit'] = true;