Harry Binswanger presents an important argument for Objectivists who dislike libertarianism. It's specific to libertarian anarchy, but I believe libertarianism leads to anarchy inevitably and so it's a good refutation of libertarianism in general.

The gist? Libertarianism is a non-philosophical movement, and as such is prone to intrincism, mysticism, and a laundry list of unsupported and often contradictory positions.