Closed PO can still be changed and invoiced

We have regular Purchase orders and service purchase orders which are completed. Fully received and fully invoiced, delivery complete and invoice complete.
In this situation, the business users expect that the system will no longer allow changes to the purchase order, no further goods receipt and no further invoices to be created.
This is not the case, we see the purchase order can be changed and follow on documents can be created against a closed PO.

Is this an error ?