Thank you all for the responses, specially Marius, Mariano, Sasi and Ankit.
Seems like the transportation lane has to be deleted manually. The external procurement relationship is deactivated and the deleted PIR will not be transfered again in APO.
After all, the way that we will handle the PIR's that are not relevant anymore will be by changing the "available to" date. From that day on the transportation lanes will not be considered in planning anymore.
For transferring the PIR changes between ERP and APO I had to activate also the CIFSRC change pointer.
Best regards,