Hello Catalin,
You can try this option,
First close the PO via MASS tcode in ECC, this should update delivery completed indicator for service PO.
Secondly, run program BBP_PO_INIT_NOTIFY_FOLLOWON with creation date of PO in SRM.
After that run program CLEAN_REQREQ_UP in SRM.
By this , ECC PO will be closed as well as no confirmation expected radio button in SRM PO will also get updated.
Regards,
Abhishek Deshpande