Remove all countries except the default PMPro country.

my-pmpro-remove-non-default-countries.php

<?php
/**
 * This recipe is an example of how to keep only the PMPro default country
 * and remove all other countries from the "Country" dropdown select.
 *
 * The PMPro default country is "US" (United States), to set
 * your custom default country, please refer to this article.
 * @link https://www.paidmembershipspro.com/change-the-default-country-of-your-membership-website/
 *
 * For available countries and their shortcode see:
 * @link https://github.com/strangerstudios/paid-memberships-pro/blob/master/includes/countries.php
 *
 * NOTE: This recipe is not intended for or compatible with the PMPro State Dropdowns Add On.
 *
 * 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_countries_show_only_default_country( $countries ) {

	global $pmpro_default_country;

	foreach ( $countries as $country_code => $country_name ) {
		if ( $pmpro_default_country !== $country_code ) {
			// remove other countries
			unset( $countries[ $country_code ] );
		}
	}
	unset( $country_code );

	return $countries;
}
add_filter( 'pmpro_countries', 'my_pmpro_countries_show_only_default_country' );