How to remove left menu items in Dashboard.

For removing submenu item: http://codex.wordpress.org/Function_Reference/remove_submenu_page

Add to functions.php

Below removes Posts, Comments, and Tools.

/**
 * Remove a top level admin menu in Dashboard
 *
 * http://codex.wordpress.org/Function_Reference/remove_menu_page
 */
function remove_menus(){
 
  remove_menu_page( 'edit.php' );                   //Posts
  remove_menu_page( 'edit-comments.php' );          //Comments
  remove_menu_page( 'tools.php' );                  //Tools
 
}
add_action( 'admin_menu', 'remove_menus' );

Before

Default WordPress Dashboard left menu

After

Edited WordPress Dashboard left menu

Resources

http://codex.wordpress.org/Function_Reference/remove_menu_page

As a plugin: http://premium.wpmudev.org/blog/how-to-remove-menus-from-the-wordpress-dashboard/