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In an Observer method, how do you tell Magento to not process code after the dispatched event
public function checkForOrdKey(Varien_Event_Observer $observer)
    {
        $controllerAction = $observer->getControllerAction();
        $request = $controllerAction->getRequest();
        $controllerName = $request->getControllerName();
        $stepData = $this->_getCheckoutSession()->getStepData();
        $ordKeyRemoved = $this->_getCheckoutSession()->getOrdKeyRemoved();
        // if it is the checkout onepage controller or inventory controller don't do anything
        if (isset($controllerName) && $controllerName === "onepage" && $stepData['shipping']['complete'] && $ordKeyRemoved) {
            $this->_getCheckoutSession()->setStepData('shipping', 'complete', false);
            $result['goto_section'] = 'billing';
            Mage::app()->getResponse()->setBody(Mage::helper('core')->jsonEncode($result));
            $this->_getCheckoutSession()->setOrdKeyRemoved(false);

        }
    }

$request = Mage::app()->getRequest();
$action = $request->getActionName();
Mage::app()->getFrontController()->getAction()->setFlag($action, Mage_Core_Controller_Varien_Action::FLAG_NO_DISPATCH, true);

$response = Mage::app()->getResponse();
$response->setHttpResponseCode(500);  //adjust to be whatever code is relevant
$json = Mage::helper('core')->jsonEncode($this->__('Your message here'));  //adjust
$response->setBody($json);
//don't need to sendResponse() as the framework will do this later