Do you wonder why GP allows you to over-receive against a PO? So do the rest of us. In my opinion, this is a shortcoming that is basic to exercising adequate control over the purchasing process.
However, if you do need that functionality in your business, Microsoft Business Solutions (MBS) Professional Services, does offer a customization (at a fairly nominal cost) they call 'PORECTOL', or PO Receipt Tolerance. This easy-to-install customization provides the ability to set a tolerance for excess receipts against your POs. You can choose a zero tolerance or some reasonable low tolerance which prevents receiving in excess of the tolerance you set.
For more information, contact MBS Professional Services through your GP Partner.