You could try a new identity under Tor, as they may have one or two high volume exit nodes banned for abuse. Lots of forums subscribe to open proxy lists as well, and ban them as a matter of course to avoid endless spamming for fake Rolex's, pyramid schemes and penis enlargement devices.Or, they may import the Tor exit list and ban all Tor exit nodes, much like imgur does cause asshole kept uploading CP over Tor, so you may not be able to use Tor to register there at all.