Are you not already using the available, built in anti-spam tools such as Recaptcha? (check the Form Properties).
There's no built-in method to black-list certain domains (like a wild-card for any address at that domain - you might not want to do that with gmail.com since there are probably lots of legit users using it.
You might have to use some other spam filtering method available through your web host (like spam-assassin or something).