Oh god that's too funny!
Especially if you read the warning message!
Maybe you should see a psychiatrist to figure out why your brain wants you locked out! LoL
Anyway, you must have done some serious acts of negligence. Either way, Windows has code that prevents administrators from completely locking out a box from any possible user, without warning them like an old nag. Security is a logical process. You shouldn't go making major changes until you know exactly, on paper, how the whole setup functions.
Inaris's solution should work. Still, I'm surprised your default administrator account is having security applied to it. Normally the admin account is setup to bypass all security to allow the account to function as it's required to.