cron currency rates & multistore

[beetea2]

I've recently set up the multistore function so that we have two versions - USA and international.

The international site has 9 currencies (including US dollars which is the default currency), but the USA site has only US dollars.

I can disable the other 8 currencies for the US site. But when the currency update cron runs (twice a day) ALL 9 currencies are re-enabled for the US site.

The cron job is run with admin/cron_currency_rates.php which updates all shops:

if (!defined('_PS_ADMIN_DIR_')) {
    define('_PS_ADMIN_DIR_', getcwd());
}
include(_PS_ADMIN_DIR_.'/../config/config.inc.php');

if (isset($_GET['secure_key'])) {
    $secureKey = md5(_COOKIE_KEY_.Configuration::get('PS_SHOP_NAME'));
    if (!empty($secureKey) && $secureKey === $_GET['secure_key']) {
        $shopIds = Shop::getCompleteListOfShopsID();
        foreach ($shopIds as $idShop) {
            Shop::setContext(Shop::CONTEXT_SHOP, (int) $idShop);
            Currency::refreshCurrencies();
        }
    }
}

If I click on the "Update currency rates" button the unwanted currencies are not re-enabled.

Any ideas on how to stop the cron job from re-enabling these unwanted currencies?