I've decided we should simply stop working out IP addresses behind proxy servers entirely. We only ever use IPs for security/anti-spam purposes, and the only IP we can trust is the IP actually in use in the connection - if we derive the IP behind the proxy it can easily be forged, and hence useless.

In this day and age people should not really be on proxies, especially as we move towards IP6. If they are being proxies then the proxy admin will have to intercede if that proxy is banned.