How to stop login from customerLogin observer

Question

I would like to deny login for a customer group. Following an example I got a working observer but I am not able to stop the login process:

public function customerLogin($observer)
{
    $customer = $observer->getEvent()->getCustomer();
    $session = Mage::getSingleton('customer/session');

    if ($customer->getData('group_id') == 4)
    {
         $session->setCustomer(Mage::getModel('customer/customer'))->setId(null);
         Mage::throwException(__('This account is not activated.'));
    }
}

Answer 1

This is pretty straight forward, you just need to unset the details stored against the customer/session.

public function customerLogin($observer)
{
    $customer = $observer->getEvent()->getCustomer();
    $session = Mage::getSingleton('customer/session');

    if ($customer->getData('group_id') == 4)
    {
        $session->setId(null)
           ->setCustomerGroupId(Mage_Customer_Model_Group::NOT_LOGGED_IN_ID)
           ->getCookie()->delete('customer');

        Mage::throwException(__('This account is not activated.'));
        return;
    }
}

Answer 2

I found something and now this works, but Luke's solution is better.

// redirect to new page
Mage::app()->getFrontController()->getResponse()->setRedirect(Mage::getUrl('index'));
Mage::app()->getResponse()->sendResponse();

// Logout
Mage::getSingleton('customer/session')->logout();

exit;

Note, that it stops working if you logout before redirecting or if you throwException().