Eliminating specific Domain Names from Registering

I’m sorry if this has been covered already – I’ve searched through the forums and haven’t found anything that really specifies what I’m looking for here in any clarity.

That said, does anyone know of a plugin to allow you to eliminate specific users from registering based on their domain name?

I get alot of [email protected] and I’m certain they’re trying to SPAM me or exploit WP in some way – at least one or two a day, and would love to just eliminate them from being able to register.

Any ideas would be much appreciated!