When blocking obvious spam on a site contact form, is it better…

Website best practices poll:

When blocking obvious spam on a site contact form, is it better to discard the message silently or kick back a 403 error?

Wayne answer on Google+: I think it’s better to give a custom message stating their action is invalid. A 403 would just be a number to most people. And wouldn’t get the message across as clearly. You need to make them aware of their wrongdoing