How could i Block Proxy Access to my website?

I was wondering if it is possible to add a code to my site that would block people from accessing my website using an online proxy?

Thanks

 

<?php

if (@fsockopen($_SERVER['REMOTE_ADDR'], 80))
{
    exit('Proxies are not allowed.');
}

?>

PHP:

That'll may block a few other users as well though. But I guess that's the only way unless you want to collect and compare all known proxy IPs.