Block specific countries on public facing pages.

drop-in.php

<?php
/**
 * Drop-in for IP Geo Block custom filters
 *
 * This file should be renamed to `drop-in.php`.
 *
 * @package   IP_Geo_Block
 * @author    tokkonopapa <tokkonopapa@yahoo.com>
 * @license   GPL-3.0
 * @link      https://www.ipgeoblock.com/
 * @see       https://www.ipgeoblock.com/codex/#filter-hooks
 * @example   Use `IP_Geo_Block::add_filter()` instead of `add_filter()`
 */
class_exists( 'IP_Geo_Block', FALSE ) or die;

function my_blacklist( $validate ) {
    // Countries blocked by all pages.
    $blacklist = array(
        'KR',
    );

    // Check if it is not blocked.
    if ( empty( $validate['result'] ) ) {

        // Check if it is in the black list.
        if ( in_array( $validate['code'], $blacklist, true ) ) {
            $validate['result'] = 'blocked';
        }
    }

    return $validate;
}

IP_Geo_Block::add_filter( 'ip-geo-block-public', 'my_blacklist' );

https://wordpress.org/support/topic/blocking-a-single-country/

Put this drop-in.php into the directory of geolocation API library, typically /wp-content/ip-geo-api/.