Redirect users from the cancellation page to the Stripe Customer Portal.

redirect_cancel_to_stripe.php

<?php

// Copy from below here...

/**
 * Prevent users from cancelling their membership or checking out for a new one.
 */
function my_pmpro_redirect_cancel_to_stripe() {
	global $pmpro_pages;
	if ( ! empty( $pmpro_pages['cancel'] ) && is_page( $pmpro_pages['cancel'] ) ) {
		PMProGateway_stripe::pmpro_billing_preheader_stripe_customer_portal();
	}
}
add_action('template_redirect', 'my_pmpro_redirect_cancel_to_stripe', 5);