WordPress 3.1 users may have noticed the admin bar at the top of the page. It’s displayed only for the logged in users.
To disable the admin bar open the WordPress admin panel, go to Users, select any user and uncheck the Show Admin Bar options.
Unfortunately this option allows you to disable the admin bar for the specific user.
In case you need to disable the admin bar completely you can use the following plugin: Admin Bar Removal (completely disable) 3.1+ only