block-po-boxes.php

<?php

/**
 * Prevent PO box shipping
 */
add_action( 'woocommerce_after_checkout_validation', 'deny_pobox_postcode' );

function deny_pobox_postcode( $posted ) {
	$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( sprintf( __( 'We Are Unable To Ship To PO Boxes. Please Enter Another Address' ) ), 'error' );
	}
}

add_action( 'cfw_update_checkout_after_customer_save', 'deny_pobox_postcode_during_ajax_update' );

function deny_pobox_postcode_during_ajax_update() {
	$address  = WC()->customer->get_shipping_address();
	$postcode = WC()->customer->get_shipping_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( sprintf( __( 'We Are Unable To Ship To PO Boxes. Please Enter Another Address' ) ), 'error' );
	}
}