"That wouldn't be a problem if this site made open-proxy IPs forbidden"
That really seems to be the only solution.
I belong to several nonpolitical forums that will only accept emails from real ISPs in order to register and post, harassment like this current situation is never a problem.
That really seems to be the only solution.
I may be wrong but it is my understanding that some ISPs give you a new address any time you restart your computer.
If so, those honest folks with that connection would be penalized.