Perhaps society would, under threat of blacklisting, induce any would-be defense agencies into contractually agreeing to open itself up to random audits, inspections, and interviews. Perhaps they would also have to agree to have all of their funds, including those funds that go toward buying weapons, ammunition, and paying the salaries of their soldiers, to be held in bank accounts that could be instantly frozen the very second that society even suspects them of intending to monopolize power.
Perhaps they would also have to have a large sum of money in an escrow account that would be used to place bounties on their own heads if they decided to become aggressive against anyone in any way. As well as getting them to contractually agree to team up with all of the other defense agencies to take out any individual defense agency that has gone rogue. Society might also decide to place a limit on the market share that any individual defense agency can have to the point of any effective cartelization being practically impossible. Surely, the possibility for a defense agency to become a monopoly is still there, but it is far less likely than the possibility for an already existent state to become totalitarian. With an already existent state, you are one martial law order away from the state being allowed to have complete control over you. But a defense agency still has all of these barriers that it has to jump over just to become a state in the first place, let alone a totalitarian one. But let’s just assume that it is inevitable that this voluntary stateless society would, under all circumstances, eventually form into a state. Even then, that is not a reason to simply give up and let the state have its way with us. It does not logically follow that we should all kill ourselves, just because eventually we’re all going to die anyway.