Dear,
For P.O items having materials, can be done by activating the check box "Delivery completed" indicator.Where as for service P.Os , the user while making service entry sheet has to make "Final entry" by clicking on 
in service entry screen.
Even for STO s also same way, here system will automatically close the P.Os if qty issued in supplying plant equals to qty received in receiving plant.
Or else you can do one thing in such cases, by amending the P.O to the received qty and value.
Regards,
Pardhu