That wouldn't be a problem if this site made open-proxy IPs forbidden.
"I don't think there's anything that can be done unless one has to use a private email to register."
You have to retrieve an auto-generated password in email to initially post here right?
Even if it's a yahoo or hotmail type email account, which logs IP's, they are most definitely using some sort of anonymous surfing IP mask.
That wouldn't be a problem if this site made open-proxy IPs forbidden.