Cancel membership level and subscription at payment gateway when a member is denied.

my_pmpro_cancel_membership_on_denial.php

<?php
/**
 * Cancel membership level and subscription at payment gateway when a member is denied.
 *
 * 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/
 */
function my_pmpro_cancel_membership_on_denial( $user_id, $level_id ) {
	// Cancel the membership level, which also cancels any associated subscription at the gateway.
	pmpro_cancelMembershipLevel( $level_id, $user_id, 'admin_cancelled' );
}
add_action( 'pmpro_approvals_after_deny_member', 'my_pmpro_cancel_membership_on_denial', 10, 2 );