Avoid user registrations with specific email provider

See this hook: register_post

add_action('register_post', 'ban_email_2334', 10, 3);

function ban_email_2334( $username, $email, $errors ){

    $result = array();

    // check if email is from the provider you wish to ban
    preg_match('/bannedprovider.com/i', $email, $result);

    if( !empty( $result )){
        $errors->add('spam_email', __( 'Some choicest abuses you might want to pass on' ));
    }

    return;

}

This will throw an error and prevent registration