How do I prevent post office boxes with Address Ninja?

Open Address Ninja from Apps in your Shopify admin. On the home page, click Block P.O. boxes. In the Countries card, choose All countries, Only these countries, or Except these countries. Add countries with the Search countries field. Under Address, choose Shipping address, Billing address, or Shipping and billing addresses. Click Turn on. If you changed a setting, the button reads Save and turn on. The rule checks both address lines for terms like P.O. box and post office box, including common misspellings. To change the error message, open the Error message card.