A long time ago back when browsers supported Java applets you could include applets in your banner ads. I worked at a place that had a product that included a chat service. The chat client was in Java and would have been quick and easy to convert into an applet.
I tried to convince the person in charge of our advertising that we should include such a chat applet in one of our banner ads, which would put everyone currently viewing the banner into an anonymous chatroom. Unfortunately he wouldn't go for it.
.. we probably can't, it is too easy to automate it, and too hard to prevent that. But would be fun.