Snippet to check for PO Box Address in Shipping Line Address 1 and 2 and throw the error if found. PluginHive Plugins : https://www.pluginhive.com/plugins/

functions.php

/**
 * Snippet to check for PO Box Address in Shipping Line Address 1 and 2 and throw the error if found.
 * Created at : 19 Dec 2018
 * Updated at : 19 Dec 2018
 * PluginHive Plugins : https://www.pluginhive.com/plugins/
 * Gist Link : https://gist.github.com/varun-pluginhive/89ae4de38f5a4f5ef7a65f2a67fa1e34
 */
add_action( 'woocommerce_after_checkout_validation', function($data) {
	$text_to_check 	= array( 'PO', 'PO.', 'PoBox' );			//	Text to check in shipping address Line 1 and 2
	$message		= "PO Box is not allowed please check your address.";

	$text_to_check	= array_map( "strtolower", $text_to_check );		// Change all the text to Lower
	$po_in_addr_1	= array_intersect( $text_to_check, array_map( "strtolower", explode( ' ', $data['shipping_address_1'] ) ) );
	$po_in_addr_2	= array_intersect( $text_to_check, array_map( "strtolower", explode( ' ', $data['shipping_address_2'] ) ) );
	if( ! empty($po_in_addr_1) || ! empty($po_in_addr_2) )
		wc_add_notice( $message, "error" );
} );