The most easy way to blow a hole in "libertarian" economics is to point out that "free capitalist markets" only exist as the effect of massive state intervention and expropriation, and point out that Stalin's Five-Year Plans were only a speeded up version of what the robber barons did in Britain and the USA. If the guy comes from a neoclassicist/Friedmanite/monetarist platform, also point out that "methodological individualism" makes no sense when dealing with real human beings, that "utility functions" are about as scientifically valid as astrology, and that - once again - the free, perfect-competition, automatically clearing market only exists anywhere because of massive state intervention. (If he comes from an Ayn Rand/Ron Paul/Austrian platform, he'll agree with you on the second point but won't care.)