This project allows SquareSpace websites to prevent users from submitting forms with public, free email accounts. However, it can be easily modified to blacklist any email domain.
This project is released under the GNU Affero General Public License Version 3
To use this project, paste the following code into your page...
<script type="text/javascript" src="https://sharptopco.github.io/squarespace-email-blacklist/email-blacklist.js"></script>
<script type="text/javascript">
$( document ).ready( function() {
initializeBlacklistListener();
});
</script>