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' );