leaky-paywall-block-email-from-register.php

<?php


add_filter('leaky_paywall_account_setup_validation', 'zeen_validate_email_blacklist', 20, 2 );

function zeen_validate_email_blacklist($errors, $fields)
{
    // edit to the domains/strings you would like to block from registering
    $blacklist = array(
        'gmail',
        'yahoo',
        'colo'
    );

    $email = $fields['email_address'];

    // edit the error message you would like to display
    $message = 'You have entered a blocked email address. Please use a different one.';

    foreach( $blacklist as $needle ) {

        if ( strpos( $email, $needle) !== false ) {
            $errors['email'] = array(
                'message' => $message,
            );
        }
    }

    return $errors;
}