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

<?php
add_action('pre_ping', 'disable_pingback');
function disable_pingback(&$links) {
  foreach ($links as $l => $link)
  if (0 === strpos($link, get_option('home')))
    unset($links[$l]);
}

Code Reference:
https://developer.wordpress.org/reference/functions/wp_generator/