How do I remove the default header from the default template?

If you do not want to edit the standard header (tpl_header.php), then in tmpl_main_page.php, you can remove the call to the header, which is the following bit of code...

<?php
/**
* prepares and displays header output
*
*/
if (CUSTOMERS_APPROVAL_AUTHORIZATION == 1 && CUSTOMERS_AUTHORIZATION_HEADER_OFF == 'true' and ($_SESSION['customers_authorization'] != 0 or $_SESSION['customer_id'] == '')) {
$flag_disable_header = true;
}
require($template->get_template_dir('tpl_header.php',DIR_WS_TEMPLATE, $current_page_base,'common'). '/tpl_header.php');?>