Ok... I think I found an easy way to do what you are requesting. Unfortunately, I am now locked out of my latest portal!!! So please only do this AFTER you launch!! lol
In your Host Settings, go to Authentication Settings and uncheck all options (DNN, LiveID, OpenID). Once you save, you SHOULDN'T be able to log in. Just make sure to remove the login button in the appropriate .ascx file (in the portal skin), so your login link won't be broken.
Robert