To disable pingbacks in Wordpress simply add the following PHP code to functions.php of your current (child-) theme.

add_action('pre_ping', 'disable_pingback');
function disable_pingback(&$links) {
  foreach ($links as $l => $link)
  if (0 === strpos($link, get_option('home')))

Code Reference:

If possible, always use a child theme so that the changes are not overwritten during the next theme update.

Disable Pingbacks

How to disable pingback in Wordpress?