Wp Block Admin allows you to enter a user defined key (set in the plugin options) that be used to enter the wp-login page. Any attempts to get to access the login screen without the secret key will result in the user being redirected back to the root url. If the user does manage to get the login screen and login, they will be redirect back to the root url aswell. Only user ID of 1 is able to be in the admin panel.
This plugin is very new and and we are sure tat there is some bugs and wanted features. To report a bug or request a feature please visit http://blog.blackbirdi.com and leave a comment.
You are free to modify this plugin how ever need be.
Screenshots
This screen shot description corresponds to screenshot-1.(png|jpg|jpeg|gif). Note that the screenshot is taken from the directory of the stable readme.txt, so in this case, /tags/4.3/screenshot-1.png (or jpg, jpeg, gif)