代码部署

实现代码，放到当前主题的functions.php中即可（这里还增加了对IP的判断，更保险）：

// 获取评论用户的ip，参考wp-includes/comment.php
function ludou_getIP() {
  $ip = $_SERVER['REMOTE_ADDR'];
  $ip = preg_replace( '/[^0-9a-fA-F:., ]/', '', $ip );
  return $ip;
}
function ludou_only_one_comment( $commentdata ) {
  global $wpdb;
  $currentUser = wp_get_current_user();
  // 不限制管理员发表评论
  if(emptyempty($currentUser->roles) || !in_array('administrator', $currentUser->roles)) {
    $bool = $wpdb->get_var("SELECT comment_ID FROM $wpdb->comments WHERE comment_post_ID = ".$commentdata['comment_post_ID']."  AND (comment_author = '".$commentdata['comment_author']."' OR comment_author_email = '".$commentdata['comment_author_email']."' OR comment_author_IP = '".ludou_getIP()."') LIMIT 0, 1;");
    if($bool)
      wp_die('本站每篇文章只允许评论一次。<a href="'.get_permalink($commentdata['comment_post_ID']).'">点此返回</a>');
  }
  return $commentdata;
}
add_action( 'preprocess_comment' , 'ludou_only_one_comment', 20);