I maintain a forum for a friend who has problems with individuals repeatedly reregistering under new nicknames to cause trouble. They all connect through the same proxy and so all have the same IP address - unfortunately so do a lot of legitimate forum users - hence we cannot just block that IP.

I know that I can visit sites such as this and it can detect my true IP address (beyond what the proxy shows it as) - how do I do this? I need to integrate this into my own site.