Pardot already provides you with some pretty great options when selecting who you want to allow to fill out your forms: Email with Valid Mail Server / Email not from ISPs and FREE email Providers
Let's there's a use case where you just need to set a list of domains that keep giving you headaches and target them specifcally... You guessed it. Javascript!
Where to put this?! Navigate to your form. Go to look and feel. Then BELOW form section. Toggle it over to code and past it in!
Let's pick a few test domains and block them from submitting the form...Any emails ending in the following will be rejected
Give it a try!