Get them to read some books on complexity theory and emergent behavior; convince them that real life is definitely a "complex system" and point out that no group of 5-6 men could possibly control the emergent behavior of such a system. There are just too many independent, autonomous agents competing to fulfill their own desires and interests. The lack of knowledge alone would be enough to ensure that such a thing couldn't happen.

Now that's not to say that there aren't cabals of people who do wield influence over certain things, or cabals of people who want to control things, or who think they control things. Take the group of bankers who run the Fed, for example... that's a fairly small group of people who have a fair amount of ability to influence things (in the US anyway). But that's still far removed from the kind of detailed control that the conspiracy theorists usually talk about.