Add this code to the functions.php
file of the active theme:
add_filter('comment_form_default_fields', 'unset_url_field');
function unset_url_field($fields){
if(isset($fields['url']))
unset($fields['url']);
return $fields;
}
When this is in place, a normal user will not see the URL field. If you get messages posted that still have a URL, you know it is SPAM and can junk it.