Purchase requisition quantity more than PO qnty

Hi

I have raised a Pr for 100 and i could make PO with referance to PR for more than 100 and to any unlimited quantity. Is it standard setting?

I am getting an warning message when i exceeded the PR quantity, the message is:

Materials of requisition 10000191 item 00010 alr. ordered in full

Message no. 06076

I require to avoid the above scenario can we set the above warning message as error? or could you guide me to avoid this by any user specific setting by functional authorisation.

Regards

Arvind