Passwords will stop them ...

Me: "Mr. Client, you have no firewall at this Internet connection point. You really need to deploy one."

Client: "Why?"

Me: "Because you currently have no protection from hackers. Anyone on the Internet can get direct access to your system."

Client: "That's why we have passwords. They have to login first, and if they don't have a password, they can't get in."

* facepalm *