control good receipt by inbound DN instead of PO.

Dear

I am not quite familiar with inbound delivery note. for now, I am using inbound DN for good receipt .  can I restrict  the good receipt only agaisnt inbound DN, instead of purchase order. 

I tested it , but after done VL31N, I still could do good receipt against POby MIGO.  what is the meaning for creating inbound delivery note if i could do GR against PO anyway.

Thanks for sharing in advance.

Best Regards

TP