How to filter PO outbound messages

Hi,

I would like to stop the following outbound messages being generated from SRM:

PurchaseOrderChangeRequest_Out

PurchaseOrderCancellationRequest_Out

I only need PurchaseOrderRequest_Out which is working fine. The other two above are failing as no receiver exists. Is there any configuration or BADI which can be employed to stop the outbound messages from being generated?

Thanks

Che