How to disable admin bar in wordpress

Set the display status of the Toolbar for the front side of your website (you cannot turn off the toolbar on the WordPress dashboard).
Placing the above line of code in theme's function.php file will prevent the Toolbar from rendering on the front end of your site.
You can also determine for which users the admin bar is shown. For example the following lines will only display the admin bar for users with administrative privileges.

if ( ! current_user_can( 'manage_options' ) ) {
    show_admin_bar( false );
}

With newer version of WordPress you may need to use the following which 
will leave the Toolbar available in the Dashboard but hide it on all 
front facing pages. 

add_filter('show_admin_bar', '__return_false');