I've been having a spam problem on my site, where people sign up and act extremely abusive to other users of my site. I can easy IP ban them, except they always come back under a different IP address through a proxy or TOR.
So I was curious if there are any php classes or functions that can look up the IP and determine if its a genuine user, or someone behind a proxy, in which case it would muzzle their accounts upon registration.
Many legitimate users will come to you through proxies - are you sure you want to filter all of them out? For example:
To answer your question, checking for the X-Forwarded-For or Via headers is probably your best bet.
Following RichieHindle's answer, I'd suggest some kind of profanity filter/detection - detect the unacceptable behaviour and suspend the accounts. Use of a proxy could definitely influence weight of decisions made by the filter/detector!
Actually stopping them is difficult, but if their nasty content doesn't get published they'll soon give up.