I think the code you need to remove is on the confirm page.
It will be near the top... on the confirm page it looks like:
<?php
while (!$WA_Store_Cart->EOF()) {
?>
<?php
$_GET['CurCartID'] = $WA_Store_Cart->DisplayInfo("ID");
?>
<?php
$IDParam_WADAProducts = "-1";
if (isset($_GET['CurCartID'])) {
$IDParam_WADAProducts = (get_magic_quotes_gpc()) ? $_GET['CurCartID'] : addslashes($_GET['CurCartID']);
}
mysql_select_db($database_localhost, $localhost);
$query_WADAProducts = sprintf("SELECT products.*, (ProductStock - (SELECT Coalesce(Sum(DetailQuantity),0) FROM orderdetails INNER JOIN orders ON OrderID = DetailOrderID WHERE DetailProductID = ProductID AND OrderDate > ProductUpdateDate)) * ProductLive AS NumLeft FROM products LEFT OUTER JOIN productcategories ON ProductCategoryID = CategoryID WHERE ProductLive <> 0 AND ProductID = %s", GetSQLValueString($IDParam_WADAProducts, "int"));
$WADAProducts = mysql_query($query_WADAProducts, $localhost) or die(mysql_error());
$row_WADAProducts = mysql_fetch_assoc($WADAProducts);
$totalRows_WADAProducts = mysql_num_rows($WADAProducts);
?>
<?php
if ($row_WADAProducts['NumLeft'] < $WA_Store_Cart->DisplayInfo("Quantity") && $row_WADAProducts['ProductUnlimited'] == false) {
header("Location: cart.php");
}
?>
<?php
$WA_Store_Cart->MoveNext();
}
$WA_Store_Cart->MoveFirst();
?>