HI,
You can try this. Try BADI BBP_ALERTING. there is a std implementation CL_BBP_CHANGE_DOC_ALERT_BADI. look at the method IF_EX_BBP_ALERTING_BADI~BBP_EVENTS_ANALYZE. The implementation is for shoppin cart. What you can do is you can create a new implementation for this BADI and inside the method write your custom logic on when to trigger the PO output. if you don't want to trigger the output remove the event SENT from CT_EVENTS table.
thanks,
sankar.