so what happens when someone decides not to?

if i choose 'no' and happen to be the largest producer of food for x region i can simply dominate. offer food to enough people to form my own militia and then only give food to those who do what i want.

you have no answer to this that isnt itself facing the same issue (the defence force is the easiest way to get your own militia, even if you didnt bribe them what if they did the same thing? were up to minor civil war now).

how do you prevent someone with resources using those resources to slowly gain control?

anarchy and libertarianism both rely on far too much hippy BS to ever function (no system ever conceived has survived the wealthy, ever)