How to also enable comments to be hidden for not selected users

@sonja, the plugin is called User Specific Content and that is way its only for content, as for comments, you can filter and control who sees them with the built-in WordPress Plugins.

UPDATE
i keep getting the same questions for people so here is a quick solution, the idea is to create a new instance of the plugin and use its filter function something like:

<?php
//get comments and comment form as a var
global $post;
$comments = get_comments('post_id='.$post->ID);
$com = '';
foreach($comments as $comment) :
    $com .= '<div class="com_container">
        <div class="com_author">'.$comment->comment_author . '</div>
        <div class="com_content">' . $comment->comment_content.'</div>
        </div>';
endforeach;
ob_start();
comment_form();
$com .= '<div class="com_form">'.ob_get_contents().'</div>';
ob_end_clean();    

//here you have two options, either to set the shortcode manually for each page:
//$com = '[O_U user_id="1,2,3" blocked_meassage="comments are closed for everyone!"]'. $com .'[/O_U]';
//then run it by the shortcode
//echo do_shortcode($com);

//or you can create a new instance of the plugin class and let it use the same sattings as the content on that page:
$limit_C = new bainternet_U_S_C();
echo $limit_C->User_specific_content_filter($com);
?>