Magento. Redirect to custom URL in observer. Close default route.

mage_redirect_to_in_observer.md

   /**
     * Redirect redirect from default customer/account/createPost action to noRoute action (404 page)
     * Observer event "controller_action_predispatch_customer_account_createpost"
     */
    public function redirectToNoRoute()
    {
        Mage::app()->getResponse()
            ->setRedirect(Mage::getUrl('cms/index/noRoute'), 301)
            ->sendResponse();
        
        die();
    }