Spammers tend to use URL field to post links to their spammy websites. Just to be safe, you may want to remove that URL field altogether.

To do that just add the code below to your theme’s functions.php file.

add_filter('comment_form_default_fields', 'unset_url_field');

function unset_url_field($fields){
 if(isset($fields['url']))
  unset($fields['url']);
  return $fields;
}