<?php
$quoteID = Mage::getSingleton("checkout/session")
            ->getQuote()->getId();

if($quoteID)
{
    try {
        $quote = Mage::getModel("sales/quote")->load($quoteID);
        $quote->setIsActive(false);
        $quote->delete();

        return "cart deleted";
    } catch(Exception $e) {
        return $e->getMessage();
    }
}else{
    return "no quote found";
}