gistfile1.php

add_action('woocommerce_after_checkout_validation', 'deny_pobox_postcode');

function deny_pobox_postcode( $posted ) {
  global $woocommerce;
  
  $address  = ( isset( $posted['shipping_address_1'] ) ) ? $posted['shipping_address_1'] : $posted['billing_address_1'];
  $postcode = ( isset( $posted['shipping_postcode'] ) ) ? $posted['shipping_postcode'] : $posted['billing_postcode'];
  
  $replace  = array(" ", ".", ",");
  $address  = strtolower( str_replace( $replace, '', $address ) );
  $postcode = strtolower( str_replace( $replace, '', $postcode ) );

  if ( strstr( $address, 'pobox' ) || strstr( $postcode, 'pobox' ) ) {
    WC_add_notice( "Sorry, we don't ship to PO BOX addresses." );
  }
}

This snippet still allows the order to go through. It just prints the message on the "order was successful" page. In order to prevent the order from going through, you need to specify that this is an error message:

WC_add_notice( "Sorry, we don't ship to PO BOX addresses.", "error" );

Hi folks, in which file should this snipped be placed?

Thanks @MrFent !
@Nallo - functions.php in your theme folder.
Now if we could just get it to highlight the address input field when it errors, that would make it complete. 👍

Ditto on the thank you @MrFent !
I just tested the code that was on WooCommerce and it still allowed the order to go through. Changing up the notice line now to test again.

Updated here: https://docs.woocommerce.com/document/dont-allow-po-box-shipping/