We absoulutely need this functionality as well! I just checked with HubSpot support, on 6/25/18, and they confirmed that the software does not have the global ability to prevent email domains on all forms. The only way to prevent form submissions from certain email addresses would be on a form-to-form basis under the ‘Email domains to be blocked’ field for the ‘email address’ form field. Please give us this feature HubSpot!