Is there a way to keep fake proxy coming to visit your server?

[CBiLL]

One of my client is having a problem of someone using some fake domain name or ip or whatever and posting his forum since it never a real address it not something he could "ban" on his forum since they just change the proxy ip whenever a ban is placed ..

Here an example he was using when he made his last post ..

proxy5.anon-online.org

which resolves to nothing as far I know so just wondering if there a way to stop those people visiting your website that are using fake ips or domains ...

I run Fedora 1 with Plesk 7.5.2 ..

Thanks ..