Laravel "forbidden" validation: fail validation when an attribute is in the request

AppServiceProvider.php

<?php

class AppServiceProvider
{
    public function boot()
    {
        // The closure will only be called if the attribute is in the request.
        Validator::extend('forbidden', function ($attribute, $value, $parameters) {
            // Always return false.
            return false;
        }, trans('validation.custom.forbidden'));
    }
}

See laravel/ideas#84