That's only true if an assurance of anonymity is stated in the terms of service for the site. Otherwise, there is no contract as you infer. The other problem for this person is that they're defacing the site and attempting to interfere with its use. That could set them up for civil or criminal sanctions if anyone bothered to pursue them. The site admin could simply ban the IP address block they're using, as in 10.0.0.XXX where everything in the .xxx range are banned.