Don't even bother. Some people are just idiots.
The late Larry MacDonald compared some liberals to a guy who walks up to you and claims he just saw a purple elephant. No matter how you may try to persuade him otherwise, it won't work because he's just too detached from reality to reason with. It's a waste of time, time that could be spent on persuading other, more cogent folks, of the truth of conservatism.
Simple, direct and to the point.