WordPress Dashboard – Restricting Access

The content of the dashboard, however, will not be the same according to the role of the user. Contributor won’t see much of the menu, for example. A subscriber can only manage its own profile.

Try to set up different users with different roles to see how the WP Admin is displayed for those users.

Then you could try to define your own users roles, change menus and layout, or even refuse access to dashboard. There’s a lot of functions and API to help you on this : check the Codex for a start : http://codex.wordpress.org/Roles_and_Capabilities and http://codex.wordpress.org/Creating_Admin_Themes