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." );
  }
}

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/