With conspiricy theorists in general it is far better to base your counter argument on specifics and in particular items that directly address their theory rather than try and generalize or use emotional reasoning to counter their claims.
When presented with specific details and logical step-by-step reasoning that directly attacks their specific claims their arguments quickly fall apart and they almost invariably resort to ad homin attacks and gratitutious insults rather than continue the debate.
I suggest reading some of Michael Shermer's works among others as a basis. But, good "lawyering" doesn't hurt.