Hi @dsl225,
I hope you are doing well today!
You can use the following mu-plugin and adjust settings to your requirements.
https://gist.github.com/wpmudev-sls/b9f6154383e0d9ed1e88c1495a1a1c91
You can adjust the following variables;
$block_emails = [];// blacklist email, separated by comma (,), ex: [‘[email protected]’,’[email protected]’].
$email_field_id -> email field id on the form you want to use
$block_msg -> message you want to show to the blocked user
You can find more information below on how to use mu-plugins.
https://wpmudev.com/docs/using-wordpress/installing-wordpress-plugins/#installing-mu-plugins
Kind regards,
Zafer