Create a file in /admin/ called delete_all_customers.adhoc.php and add the following to it.

define('IS_MULTILANGUAGE', 1);

require './auth.php';
require $xcart_dir.'/include/security.php';

x_load('backoffice');
		
$customers_sql = "select id from $sql_tbl[customers] where usertype='C' and login!='youremail.co.uk' limit 5000";
$customers = func_query($customers_sql);

if (is_array($customers)) {
	foreach($customers as $c) {
		func_delete_profile($c['id'], 'C', false);
	}
}

echo "<pre>";
print_r($customers);
echo "</pre>";

die('end');

You can then adjust the limit 5000 for how many at a time to delete to stop the server freezing up and change youremail.co.uk to an email you do not want to delete.