Source: http://www.prestashop.com/forums/topic/9473-module-force-currency/
Note: Error in amount shown in store owner email but is correct in customer email. Use with caution.
in init.php before:
$currency = Tools::setCurrency();
add:
$cookie->id_currency = intval(Configuration::get(‘PS_CURRENCY_FRONT_DEFAULT’));
in admin/tabs/AdminCurrencies.php after:
‘PS_CURRENCY_DEFAULT’ => array(‘title’ => $this->l(‘Default currency:’), ‘desc’ => $this->l(‘The default currency used in shop’), ‘cast’ => ‘intval’, ‘type’ => ‘select’, ‘identifier’ => ‘id_currency’, ‘list’ => Currency::getCurrencies()),
add:
‘PS_CURRENCY_FRONT_DEFAULT’ => array(‘title’ => $this->l(‘Default currency front:’), ‘desc’ => $this->l(‘The default currency used in front shop’), ‘cast’ => ‘intval’, ‘type’ => ‘select’, ‘identifier’ => ‘id_currency’, ‘list’ => Currency::getCurrencies()),
Next go to admin->payment->currencies and set up your default shop currency. Customer CAN change the currency if needed.