How do you prevent monopoly capitalism under a minimalist state?

The answer is: you don’t — which is “working as intended” to a libertarian. Both because their contempt for the government is so great they would sacrifice our very national treasures if it meant squeezing another cent out of the public coffers, and because they are fans of monopoly capitalism. Greed is good — so what if the invisible hand must be made to slap around the little guy? Money is a religion, and these people are True Believers.