In the observer class enter the following code, keep an eye on the redirect, which is “checkout/cart”. You can redirect to any page or any url like <frontname/controllername/action>
Mage::getSingleton('core/session')->addError('Email is spam!');
Mage::app()->getFrontController()->getResponse()->setRedirect(Mage::getUrl('checkout/cart'));
Mage::app()->getResponse()->sendResponse();
exit;