wc-dont-allow-po-box-shipping.php

/**
 * Prevent PO box shipping.
 */
function wb_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( sprintf( __( 'Sorry, we cannot ship to PO BOX addresses.' ) ), 'error' );
	}
}
add_action( 'woocommerce_after_checkout_validation', 'wb_deny_pobox_postcode' );