Remember to take shitposts seriously, it’s what the cool kids are doing
Short answer: The community.
In small contexts, a mutual understanding is sufficient. There are “Radical Therapy” groups with no central therapist who decides who talks how much but instead have rules like fixed times for each person. I don’t think people will break these rules but exclusion is always an option with very intransigent people.
In bigger contexts like the Commons, people deliberate on their own rules. Minor transgressions will have minor consequences and the worst is – again – exclusion. People are more willing to stick to the rules and watch others if they were part of the process that created the rules. If you want to dive deeper, I remember a podcast episode by SRSLY WRONG and a YouTube video by Andrewism about The Commons or The Tragedy of the Common.
Publicly shun people. You’re a rule breaker? You’ve been shunned by society and people who associate with you will be known associates of the shunned.
And further to that we have voluntary prison. Essentially, if you’re guilty of something and want to have the benefits of this society, you need to agree to a loss of some privileges - in whatever form is necessary. If you won’t, well good luck surviving when nobody will trade with you or let you live near them.
If you won’t agree to that, you can leave, but the full details of your trial and conviction are public and your decision to leave will be broadcast, so our neighbours know to look out for you.
That means trials will need to be fair, and seen to be fair, or else it will be easy to ask for asylum. Prisoners need to be fairly treated, or they will try their luck in a nearby place.
But if someone chooses to leave and is just trying to run from the consequences of their actions, well they’ll have a hard time being accepted anywhere else.
So what do you do to deal with the situation we see in modern states with an actual centralised “monopoly” on violence: Organised criminal environments that live off exploiting the rest of society?
We’re talking about people that don’t care if you shun them, because they have their own environment, with their own hierarchy and set of rules, and they’re willing to use violence to exploit the rest of society to make a living.
Rules are enforced by the collective not by a small minority essentially. Things like direct democracy doesn’t contradict with their philosophy. Essentially middle management and above in all aspects of financial and political life would be abolished.
Direct democracy doesn’t only not contradict with anarchism, it is a core tenet of anarchism. After all, how do we get rid of unjustified hierarchy without creating a hierarchy free from rulership?