Is there any way to block the PO , so that no GR can be possible?
Is it available on header level?

Solution:

There are three ways to close/block purchase order:

1. Change P.O Quantity equal to delivered quantity.

2. Select line item in p.o, click lock icon to block it.

3. BADI for delivery completion indicator.
 

Blocking is not possible at header level.

To prevent a GR posting, you can proceed as follows:

1) Reduce the purchase order quantity to the previous GR quantity.

2) Block the purchase order item.
(ME22N: Block pushbutton; ME22 Menu -> Edit -> Block)
Fill the date field for the 'Latest Possible Goods Receipt' (LEWED) in the purchase order item. If you do this and you enter a GR document after this date, the system issues message M7163, which you must set as an error message in Customizing.

See OSS Note 856279 - MIGO: Proposed qty despite 'delivery completed' indicator

Notes:

By the way, setting the block is not really recommended and should just be a short term solution, as it will stop you from archiving this PO and not really close the business case. and even blocked POs are still considered in MRP.

See also
Explanation Of Blanket Purchase Order

Get help for your SAP MM problems
SAP MM Forums - Do you have a SAP MM Question?

SAP MM Books
SAP Material Management Certification, Interview and Configuration Reference Books

SAP Material Management Tips
SAP MM Configuration Tips and Materials Management Discussion Forum

Best regards,
SAP Basis, ABAP Programming and Other IMG Stuff
http://www.erpgreat.com