Magento – restrict user to place order if ZIP (PIN/Postcode) code is not from pre define ZIP (PIN/Postcode) code list

That is definitely a kind of a bug-feature that was done by core team on purpose, but I don't think that it is done specifically for resetting an address. Might be it was a bug fix of filled in address fields on some blocks, before onepage template has been separated from multi shipping checkout.

But the main point is that address is actually is not reset. Problem lies in the block for address steps, as if customer is not logged it it uses empty new model as data source for address form. That's simple as that. Just add observer on before_to_html for that block and set original quote address into the block and your issue is fixed.

You can use sales_order_place_before event to stop these zip codes.

In your config (this is part of config.xml)

<config>
    <frontend>
        <events>
            <sales_order_place_before> 
                <observers>
                    <stop_placing_order>
                        <class>your_module/observer</class>
                        <method>stopOrder</method>
                    </stop_placing_order>
                </observers>
            </sales_order_place_before>
        </events>
    </frontend>
</config>

In your observer you can have this function:

public function stopOrder($observer)
{
    $order = $observer->getEvent()->getOrder();
    $address = $order->getShippingAddress();
    $postCode = $address->getPostcode();

    $listOfCodes = array('2144', '456', '555');

    //now your condition
    if(in_array($postCode, $listOfCodes)){
       Mage::getSingleton('checkout/session')->addError('This Postcode is blocked');

       Mage::app()->getFrontController()->getResponse()->setRedirect(Mage::getUrl('checkout/cart'));
       Mage::app()->getResponse()->sendResponse();
       exit;
    }
}

I haven't tested but this should do the trick for you.