Completely remove ability to create users?

Okay, so the code above does work, but not for super admins, you must explicitly remove permissions from super admins, like this:

// restrict super admin capabilites
add_filter('map_meta_cap', 'restrict_super_admins', 10, 4);
function restrict_super_admins($caps, $cap, $user_id, $args){
    $super = array(
        'add_users',
        'create_users',
    );
    if( in_array($cap, $super) ) {
        $caps[] = 'do_not_allow';
    }
    return $caps;
}