Eagle eye, I registered for the forum, read through some of the posts regarding Jews. There is nothing you can do to shut them down unless they are engaging in illegal activity. They do have freedom of speech. Heck there are plenty of forums that do much worse and they aren't shut down, there are pedophile communities, snuff communities, anarchist communities, communist communities, etc. Posting ideas or beliefs (or in their case not believing the holocaust happened) it's self is not a crime in this country. Now there are cases forums have been shut down (some of the pedofile communities for posting actual child pornography) but courts ruled that it was legal for them to discuss whatever they wanted basically. Totse has people posting guides how to build bombs and break into homes and what not and they haven't been shut down, I think they've been around since 1997.
Also it appears they are hosting their own site, not from vbulletins servers. They are just using vbulletin for their forum software.