Registration Denial of Service Attack

[meissenation]

I've been seeing what appears to be a denial of service attack by flooding my website with registrations. The vbStopForumSpam log shows a bot doing "allowed registration" every 3 seconds.

Are there any products out there which can block people from trying to flood registrations?

And if not, are there any products out there that can block e-mail domains from attempting to register? I know this is a weak countermeasure since all they have to do is change the domain they're attempting to register with but thus far the bot they're using appears to only use one domain for the registration e-mail address.