Magento – How to Remove Old Cart product at loggin time ?

/app/code/local/Namespace/Cart/etc/config.xml

<events>

    <load_customer_quote_before>
        <observers>
            <module_load_customer_quote_before>
                <class>Namespce_Cart_Model_Observer</class>
                <method>clearCarts</method>
            </module_load_customer_quote_before>
        </observers>
    </load_customer_quote_before>
</events>

</events>

/app/code/local/Namespace/Cart/Model/Observer.php

<?php

class Namespace_Cart_Model_Observer extends Varien_Object {

    public function clearCarts(Varien_Event_Observer $observer) {
        $lastQuoteId = Mage::getSingleton('checkout/session')->getQuoteId();
        if ($lastQuoteId) {
            $customerQuote = Mage::getModel('sales/quote')
                    ->loadByCustomer(Mage::getSingleton('customer/session')->getCustomerId());
            $customerQuote->setQuoteId($lastQuoteId);
            $this->_removeAllItems($customerQuote);
        } else {
            $quote = Mage::getModel('checkout/session')->getQuote();
            $this->_removeAllItems($quote);
        }
    }

    protected function _removeAllItems($quote) {
        foreach ($quote->getAllItems() as $item) {
            $item->isDeleted(true);
            if ($item->getHasChildren()) {
                foreach ($item->getChildren() as $child) {
                    $child->isDeleted(true);
                }
            }
        }
        $quote->collectTotals()->save();
    }

}