Event Tickets Plus: WooCommerce: Force all tickets to be "Sold Individually".

functions.php

<?php
/**
 * Event Tickets Plus: WooCommerce: Force all tickets to be "Sold Individually".
 *
 * By default, this limits the purchase quantity to 1, but this is quantity is
 * filterable with `woocommerce_add_to_cart_sold_individually_quantity`.
 *
 * @see Tribe__Tickets_Plus__Commerce__WooCommerce__Main::get_event_for_ticket()
 * @see WC_Product::is_sold_individually()
 * @see WC_Cart::add_to_cart()
 *
 * @link https://gist.github.com/cliffordp/9a457b724e38b3036f8d48adc90930ed
 */
function cliff_et_force_woo_tix_sold_individually( $sold_individually, $wc_product_instance ) {
    if (! class_exists( 'Tribe__Tickets_Plus__Commerce__WooCommerce__Main' ) ) {
        return $sold_individually;
    }

    $wootix = new Tribe__Tickets_Plus__Commerce__WooCommerce__Main;

    if ( empty( $wc_product_instance->get_id() ) ) {
        return false;
    }

    if ( $wootix->get_event_for_ticket( $wc_product_instance->get_id() ) ) {
        return true;
    } else {
        return $sold_individually;
    }
}

add_filter( 'woocommerce_is_sold_individually', 'cliff_et_force_woo_tix_sold_individually', 10, 2 );