WordPress MU

I do not know if there is an add on or a feature already available but I've been getting quite alot of signup spam.
This one guy seems to love spamming http://www.iktib.com with Toyota Scion blogs and not doing anything with them (I'm sure to link to his site and bump it up).

Is there a way to block a domain so that any email address from that domain is automatically rejected?

Yes.

Login as Siteadmin then go to

site admin -> Options = partway down the page is where you set the restrictions for the entire install.

excellent thank you

here is the specific section for future people

Limited Email Registrations