Prevent P.O Box Addresses

How does one prevent customers from adding po box addresses in the customer and shipping options?

All we did was to post some verbiage on the checkout page, we've never had a problem. 1 out of a 1000 you might have to call and get a new shipping address. There are probably services out there like the post offices CASS certification that can do that for you automatically.

"Shipping is NOT available to PO Boxes. Shipping available within Continental US only."

That will have to do, thanks for the quick reply. Which template do I have to edit for this?

edit: discovered how to do it

I would love to know how to get a fix for this in version 3.0.2 - I have done as you said above but still 3/10 orders still put in a PO BOX.