remove admin bar new post/link/media sub menu

See the Admin Bar remove_node function:

add_action( 'admin_bar_menu', 'remove_wp_nodes', 999 );

function remove_wp_nodes() 
{
    global $wp_admin_bar;   
    $wp_admin_bar->remove_node( 'new-post' );
    $wp_admin_bar->remove_node( 'new-link' );
    $wp_admin_bar->remove_node( 'new-media' );
}