The gist below will stop a user from signing up if their last PMPro order on record was refunded.

pmpro_user_last_order_refunded_check.php

<?php
/**
 * pmpro_user_last_order_refunded_check The gist below will stop a user from signing up if their last PMPro order on record was refunded.
 *
 * Add this code to your PMPro Customizations Plugin
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 *
 */
function pmpro_user_last_order_refunded_check( $okay ) {

	// If things aren't okay to start with, let's bail.
	if ( ! $okay ) {
		return $okay;
	}

	global $current_user;

	// See if the user's last order on record was 'refunded', if so, stop checkout process.
	if ( ! empty( $current_user ) ) {
		$order = new MemberOrder();
		$order->getLastMemberOrder( $current_user->ID, 'refunded' );

		if ( ! empty( $order->id ) ) {
			pmpro_setMessage( 'Please contact us about your account to complete checkout.', 'pmpro_error' );
			$okay = false;
		} else {
			$okay = true;
		}
	}

	return $okay;
}
add_filter( 'pmpro_registration_checks', 'pmpro_user_last_order_refunded_check' );

This recipe is included in the blog post on "Restrict Checkout for Users with a History of Refunds" at Paid Memberships Pro here: https://www.paidmembershipspro.com/restrict-checkout-for-users-with-a-history-of-refunds/