We are proceeding from a flawed premise, that there is force A (just) and force B (unjust) when a Free society would likely entail forces A through ZZ. If Force B begins to behave like a group of thugs, chances are they are going to upset many more groups than just belong to force A, and in doing so will eventually be put down (I am borrowing from Rothbard here, almost verbatim). Even if Force B is better funded because they cater to rich groups, that advantage in funding will be equalized when say Force A,C, and D all decide it is time to cooperate in putting down Force B (because, taken as a net A,C, and D will likely have equal funding to Force B). The idea is that it is better to be able to choose than to be arbitrarily forced to support Force B.
The only way to opt completely out of society is to opt out of life, which makes all of this a non issue. Furthermore, you talk about Force B as though it grew to national power overnight... this is unlikely unless, as is the current case, everyone is FORCED to fund them. What is more likely is that people, understanding strength in numbers and the value of efficiency, will group together and contract Security Forces on a state to state, county to county, or even on a town to town basis.
However, just because a large number of individuals have chosen to group together for purposes, a fore mentioned, does not mean that an individual that feels his liberty is being infringed by the group's chosen security firm should be a party to his own persecution. It is very likely that in a society where security is provided by free market enterprises, but the rule of law is provided by the Constitution, specialist groups will arise that offer protection to these individuals because there would be nothing to keep the individuals, no matter the physical difference between them, from banding together themselves out of the same mutual instance of protection that would lead to state wide, county wide, or town wide security forces.
Furthermore, A free society would also entail free travel, so if a local force began to persecute and individual unfairly (perhaps because his morality did not click with the larger group) then that individual could, and probably would, to say nothing of should, leave and go to a place where people that share his morality have congregated. If he likes screwin barn animals, he will likely have trouble finding a state, county, or town that will cater to him sufficiently. But if he is: Gay, a drug user, a socialist, likes to walk on his hands, or is just a loner then chances are he will be able to find a place where people share his morality, and employ a security force (which are likely to be made up of locals as it would be much less expensive, not the corporate stuff everyone envisions) that will not persecute him.
Forgive my rant, I am still working through this concept myself.