Hi Rajasekhar,
There is no any option to delete purchase order from data base.
You have option to Block or Delete the line item of purchase order.
you can maintain the Del flag for the line items in PO.
Check the below link for your reference.
http://scn.sap.com/thread/1682393
http://scn.sap.com/thread/1578614
Regards,
Bastin.G