<?php
//cancel coupon code
$coupon_code = Mage::getSingleton('checkout/session')
->getQuote()->getCouponCode();
if ($coupon_code) {
Mage::helper('checkout/cart')->getQuote()
->setData('coupon_code', '')->save();
/** or use Mage::getModel
*Mage::getModel('checkout/cart')->getQuote()
->setData('coupon_code', '')->save();
*/
}