Ban proxies?

[T4doom]

Recently I've been experiencing some abuse on my boards. This was done mainly by users accessing my boards through a proxy.

Now, so far I've banned over 30,000 web proxies by hand. And over 100,000 public and private proxy lists.

Yet STILL, they return.

I actually KNOW there is a way to ban almost all common proxies, like the board at http://www.sythe.org has. They managed to put a proxy detection system into place that filters out just about all the proxies you try to access it with,

So I was wondering if anyone had an idea how to re-create that/put up a good protection of my own?

Any help is of course greatly appreciated.