This question has been answered by multiple community members. See the first response.
[[StopBadEmail]] <form id='my-form' ... >
/* StopBadEmail snippet */
$js = "
document.getElementById('my-form').onsubmit = function() {
var substring = 'printemailtext.com';
var email = document.getElementById('email').value {
return email.indexOf(substring) === -1? true : false;
};"
$modx->regClientStartupScript($js);