I found a solution to add rel="nofollow" to comments. My theme maker  gived me this hack that
 you can add in fucntions.php (Dont garantie that will work for evryone depend of your theme)

    function add_nofollow_to_comments_popup_link() {
    return ' rel="nofollow" ';
    }
    add_filter( 'comments_popup_link_attributes', 'add_nofollow_to_comments_popup_link' );