Example on how you can prevent registrations if the email address has already been used on the event.

tw_ee_attendee_information__process_reg_step.php

<?php // Please do not include the opening PHP tag if your already have one.
function tw_ee_attendee_information__process_reg_step($process_reg_step, $reg_step, $spco) {
    //Stop processing for testing
    //$process_reg_step = false;
    // Pull the transaction from checkout.
    $transaction = $spco->checkout->transaction;
    if($transaction instanceof EE_Transaction) {
        // We need to process the registrtion data, to do that we need the registrations
        $registrations = $transaction->registrations();
        // Now process the data from the reg_step
        $valid_data = $reg_step->valid_data();

        $personal_info_identifier = EEM_Question_Group::instance()->get_var(
            array(
                array( 
                    'QSG_system' => EEM_Question_Group::system_personal 
                ), 
            ),
            'QSG_identifier'
        );
        //Loop over each question group assigned to the primary registrant and pulls it's questions.
        foreach($registrations as $reg) {
            if( isset( $valid_data[ $reg->reg_url_link() ][$personal_info_identifier]) ) {
                $email = $valid_data[ $reg->reg_url_link() ][$personal_info_identifier]['email'];
                $count = EEM_Registration::instance()->count( 
                    array(
                        array(
                            'EVT_ID' => $reg->event_ID(),
                            'Attendee.ATT_email' => $email
                        )

                    )
                );
                if( $count ) {
                    $error = 'SPCO has been stopped before registrations have been processed';
                    EE_Error::add_error(esc_html__($error, 'event_espresso'), __FILE__, __FUNCTION__, __LINE__ );
                    return false;
                }
            }
        }
    }
    return $process_reg_step;
}
add_filter('AHEE__Single_Page_Checkout__process_reg_step__attendee_information__process_reg_step','tw_ee_attendee_information__process_reg_step', 10, 3);