Block Certain Emails from Contacting through a Form

Hey Guys!

Does anyone know what snippet of code would allow it to be possible to block certain email addresses from contacting you from a contact form?

Have a client that is getting the same spam emails over and over from the same email and I would like to find a way to stop it at the WordPress level before the email is ever sent.

Can't believe that I haven't found a plugin that already does this or at least baked into a contact form plugin already. Thanks!