Standard Oil had a great many competitors, until Rockefeller bought them all. Bezos has built his monopoly at Amazon because he bought anyone who was a competitor, same for all the oligopolies in America currently. Phizer is a marketing company that buys up smaller companies who make pharmaceuticals. Granted today we see seamlessness between gov and corp, but why would that be different without gov? That is the historic reason for Jubilee, because eventually everyone is owned.

As for policing, I mean it only in the most general sense. I'm not sure what other word would fit, for how to tangibly prevent a free market from becoming a monopoly.