So if in an ancommunist/ansyndicalist society somebody murders someone for no reason will he be punished...

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Well it's not heirarchical

and it's voluntary

i can't imagine sole individuals as the final determiner of when someone did something violent, and punishment of some kind justified
maybe direct democratic vote? jury?

was meant for

How will the people decide under what circumstances to punish somebody?

There will have to be some kind of court system in any successful society, or otherwise you risk killing the innocent.
A court system will require laws/rules set in place for a community/organization of people.

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Anarchy isn't "no rules". It means no rulers.

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It's like you ignore not just everything the left says, but everything right libertarians say as well.