In admin->system->user info settings you can set "allow new user registration" to "no". Then turn off the login block. (invisible)

For you to administer your site you go directly to the user.php page to login. Then you can either type in the admin.php page, or make the user menu block visible to webmaster group only. You as administrator must be able to login to reach admin and this is how you can do it.

Make sure you make all your modules and blocks visible for anonymous group and there you have it. No site levels, all public, except you the admin.