In order to remove "login" form on checkout page, you can try to implement following solution:
http://excellencemagentoblog.com/blog/2011/10/17/magento-onestep-checkout-remove-login-step/
Another method which can help you to achieve the similar effect is about making an event observer on customer_registration_is_allowed event, and put following code
public function disallowRegistration(Varien_Event_Observer $observer)
{
$observer->getResult()->setIsAllowed(false);
}
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