The only way around that is for the person who runs hte proxy to do a routing with IPtables and use a different IP for outgoing traffic with port 80 firewalled.
Of course that is not common.
You can also check things like their Adsense pub on their page, the IP their website is hosted on. Their DNS server. You could lock the submissions by their account to some amount everday, have accounts require email validation, Dont allow account to be created from the same email, dont allow account tobe created from the same IP withint 7 days, (You can store a time() in the db and compare it)
So to create new accounts, they would have to create email after email to make new account, while still trying to avoid the other barriers such as the IP. It becomes a lot of hoops to jump threw.
If someone manages to get threw all that, then they would pretty anonymous anyways in the list and you wouldn't know.
I think that it would definitely would end up being a complicated algorithm, so it definitely put your programming skills to test to create it. :P