For the Invite Only Add On, set the maximum times a code can be used to only once.

my-pmpro-invite-code-max-usage-1

<?php
/**
 * For the Invite Only Add On, set the maximum times a code can be used to only once.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

// Set the maximum times a code can be used to once.
define( 'PMPROIO_CODES_USES', 1 );