add_filter('pre_comment_on_post', 'verify_spam');
function verify_spam($commentdata) {
$spam_test_field = trim($_POST['comment']);
if(!empty($spam_test_field)) wp_die('Спаму нет!');
$comment_content = trim($_POST['real-comment']);
$_POST['comment'] = $comment_content;
return $commentdata;
}