WordPress: No comment URL -- or die

gistfile1.php

<?php
/*
Plugin Name: Ozh' No Comment URL
Plugin URI: http://planetozh.com/
Description: No comment URL or die
Version: 1.0
Author: Ozh
Author URI: http://ozh.org/
*/


add_filter( 'preprocess_comment', 'ozh_ncu_check', -999 );

function ozh_ncu_check( $in ) {

	// Don't check logged in users
	global $user_ID;
	if( isset( $user_ID ) && $user_ID )
		return $in;

	// Die if comment URL
	if( !empty( $in['comment_author_url'] ) ) {
		wp_die( 
			"<h1>Sorry, you don't qualify to comment on this site</h1><p>Go spam somewhere else.</p>",
			"Forbidden",
			array( 'response' => 403 )
		);
		die();
	}
	
	// So far, so good
	return $in;
}

gistfile2.md

In the theme CSS, something like:

#comment_url {
    display:none;
}