PO Delivery Settings

Dear Experts,

I've Created the PO for 100 units... and i made GR for 50 units.

At this point of time.. i've gone to PO change and make the delivery Completed indicator as checked..in DELIVERY tab

Now the status of the PO become '0'..

But i tried to make GR for that PO, it is allowing.. please let me know is this correct functionality... ???

How can i short close the PO... as No balance to receive.. also NO further GR to be allowed.. Please explain in this regarding..

Thank you.