Well, this thread has drifted pretty far off topic.
Blask's view is classic; the state enforces the social contract and protects the commons.
The question of what stops a plutocracy from forming is much simpler than it appears. To look deeper, ask how the people would get rid of a plutocracy in the first place. The answer is communal self defense, based on an educated and empowered population. You have to develop that before anarchy can break away from plutocracy in the first place, and it is that same empowered population that would prevent a plutocracy from reforming.
But when you start to examine what sort of structures a people free of plutocrats would build to continue to enforce the social contract and protect the commons from plutocracy, it starts to look an awful lot like a state. The difference would be voluntary association based on enlightened self interest. When you ask the question, what would be done about people who enjoy the benefits of an enforced social contract, but refuse to participate, things start to fall apart.
In my view, society is not ready for anarchy. People are too selfish. We could change, over time. The zeitgeist progresses. Without socialization, it would be Lord of the Flies. Parents have a social responsibility to instill values in their children, and there are social consequences for breaking those values. One of those values could become voluntarily enforcing the social contract. Then the answer to what prevents the plutocracy becomes socialization.
One of the most distasteful aspects of the state is corruption. It is the coercive power of the state combined with an uneducated, disempowered population that allows for corruption. Corruption is not binary, it is analog; there are degrees. Thus anarchy can be seen as the edge of a spectrum; the limit as corruption approaches zero. To promote anarchy can be seen as the fight to hold power accountable. It is the skills that people learn and the alternatives to power that people create in the process that will prevent plutocracy from reforming.
If power is the ability to control other people's lives, than in order to create freedom, one must destroy power.