This is fairly simple, as private defense agencies do not operate in a vacuum. I imagine there would be a form of natural checks-and-balances, specifically in the form of militias and community watches/defense.
It would be impractical for a defense agency to use violence against others to force service consumption, as the very act would be the impetus for people to defend themselves and fight back otherwise.
The first line of defense is the individual level, with home security measures and one's community, whether from friends, family, church, or other social/civic association.
I'm convinced about the human ability to wisdom, great virtues and having understanding of politics, but ... these advantages usually apply to a small percentage of people....
It bothers me a little.
But well .... education, education and education - this seems to be a very important part in our fight for Free Society.
Support of the common poeple is the biggest power that makes egsistence of the state possible, so maybe also support of the common people would be the biggest power that would make the egsistence of the Free Society possible to.