Ah, thank you. So you are concerned that in an anarcho-capitalist society there would be nothing to prevent a powerful, malevolent hierarchical organization from assigning itself power and taking over. This is a valid concern. Let me ask you a question before I attempt to respond: What stops a government (even a democratic or constitutional republic) from becoming a dictatorial and draconian nightmere oppressing and enslaving its populace?
To make it more simple; what stops an organization becoming so powerful that it can use coercion to enslave the rest ?
So ??
Yes, as Myrkul says, the other ones. Governments can afford to have large armies because they can forcibly take money from their citizens and amass large amounts of wealth with which to muster and pay armies. Private companies will have a harder time doing this, and will be subject to the laws of the market. Namely, those that do not best serve the consumers will suffer losses, those that best serve the consumers will get profits. Private companies who start building armies and become aggressive will lose customer support and will not be able to afford to keep their armies.