To hide pligg registration:
Edit lib/html1.php
in Function force_authentication()
Replace it with the following:
global $current_user;

    if(!$current_user->authenticated) {
       
        echo '<div>You must first login to access this section. Please <a href="http://www.site.com/index.php?option=com_login&task=edit&id=10" target=_parent>login</a> or <a href="http://www.site.com/index.php?option=com_comprofiler&task=registers" target=_parent>register</a> here.</div>'; //header("Location: " . getmyurl('login', $_SERVER['REQUEST_URI']));
        //header("Location: " . getmyurl('login', $_SERVER['REQUEST_URI']));

        die;

    }

    return true;