Disallow img tag in comments?

You can strip images on display pretty easily.

add_filter(
  'comment_text',
  function($comment) {
    $allowed_html = array(
        'a' => array(
        'href' => array(),
        'title' => array()
      ),
      'br' => array(),
      'em' => array(),
      'strong' => array()
    );
    return  wp_kses($comment, $allowed_html);
  }
);

That will strip any tags not listed in the provided array. To specifically strip images with links to external sources you need something more complicated.

function strip_external_images($match) {
  if (empty($match)) return;
  $site = parse_url(get_site_url());
  $parsed = parse_url($match[1]);
  if (empty($parsed['host']) || $site['host'] !== $parsed['host']) {
    return '';
  } else {
    return $match[0];
  }
}

add_filter(
  'comment_text',
  function($comment) {
    $pattern = '|<img.*src="https://wordpress.stackexchange.com/questions/113762/([^"]*)"[^>]+>|';
    return preg_replace_callback($pattern,'strip_external_images',$comment);
  }
);

That should allow users to add images to comments but only images hosted at same domain as the site itself.