mu_pmo_order-validation.php

/**
 * Blocks order if condition is met
 * - blocks Emma Rose orders
 * - blocks orders from New York
 *
 * @param array $data
 * @param object $errors
 * @return void
 */
function pmo_custom_checkout_field_validation( $data, $errors ) {
    if( strtolower($data['billing_first_name']) == "emma" && strtolower($data['billing_last_name']) == "rose" ) {
        $errors->add('validation', 'Sorry, Emma Rose is not allowed to make orders. If you think this is an error, contact us by email.');
    }
    if(strtolower($data['billing_city']) == "new york") {
        $errors->add('validation', 'Sorry, New York is too far for us to deliver, find a dealer closer to your location.');
    }
}
add_action( 'woocommerce_after_checkout_validation', 'pmo_custom_checkout_field_validation', 10, 2 );