Maybe we have a different understanding of anarchy.
For me anarchy means NO RULER over me. You can have RULES though which you agree upon, or if not then yoy are free to leave the space and find another with a set of rules you can agree with. That is anarchy and a free society.
Who enforces the rules you agree upon?
The respective parties. If you don't want to be in that agreement anymore, you move out of it. You enforce it by adhering to it. Just like bitcoin.
You consent to the rules upfront (unlike being born to a state that then keeps adding amd changing rules without your explicit OK)
If someone breaks the rules it can also be dealt with, eg. with arbitrage.
I'd recommend to read Paul Rosenberg's "Justice Without Force". I can share the doc with you if you have trouble finding it.
