Disallow PO Boxes in billing and/or shipping addresses.
To disable PO Boxes in billing addresses, go to the admin in System -> Configuration -> Customers -> Customer Configuration -> Name and Address Options, and set Disallow PO Boxes in Billing Address to Yes:
To disable PO Boxes in shipping addresses, go to the admin in System -> Configuration -> Sales -> Shipping Settings -> Options, and set Disallow PO Boxes in Shipping Address to Yes:
On the front end:
