You can cancel your pre-order item(s) as long as these items are not in the process of being prepared to be disapatched.
If you've placed your order using a registered account, log in to your account. Navigate to your Order History, click into your order, select "Manage Order", and "Cancel Pre-Order".
If you've placed your order as Guest, go to your confirmation email, and click the link provided under "Do you need to update your order?". Select "Manage Order" and cancel the item.