Why do you want to stop people from downloading your content? Some people only use gmail/yahoo/outlook, so instead of letting them learn about you or get your trial, you are causing friction and will most likely cause them to abandon the process. What is more important: clean email addresses or leads looking to learn about and possibly buy your product?
- Forms 2.0 JS API Documentation: http://developers.marketo.com/rest-api/assets/forms/examples/
- Search for "RegEx free email accounts"
- Write a function that fails validation if the email address matches the regex test.