How do I remove the header completely

[gjh42]

There is not an admin switch for this, but you can easily accomplish it by editing /includes/templates/your_template/common/tpl_main_page.php.
After the information comments at the top of the file, add

$flag_disable_header = true;

to get

 * @package templateSystem
 * @copyright Copyright 2003-2006 Zen Cart Development Team
 * @copyright Portions Copyright 2003 osCommerce
 * @license http://www.zen-cart.com/license/2_0.txt GNU Public License V2.0
 * @version $Id: tpl_main_page.php 4886 2006-11-05 09:01:18Z drbyte $
 */
  $flag_disable_header = true;