Deleting Custom Fields - still shows in invoice

If you have a custom field and then delete it, it should not show up for future orders. We had a "notes" custom field a long time ago and deleted that. Now when we copy an order, that field shows up on the invoices and can not be changed because the field has been deleted. We have to re do each line item in order to work around this issue. Would be great if the custom field is deleted, to not be able to see it again.