You know that's basically the same thing - central entity with overarching authority exerts its will over the market, canceling transactions it's deemed 'fraudulent' between two independent consenting entities. "Free hand of the market" ain't present here, chief - this is regulation.
Please don't conflate the free market with anarchy.
You should read up on the foundations of anarchy before spouting your mouth off, since you are woefully ignorant on the subject. Bakunin, Kropotkin, and Bookchin are all good places to start.
Coercion does not have to be done by a governmental entity. The philosophy of government can wax as poetic as it wants but I know too many people who would happily take advantage of anarchy to employ said coercions. Practically speaking, Human nature being what it is, coercion will be rampant in an anarchic society. I'm quite familiar with the foundations of anarchy. I just occasionally use poor wording :-)
MtGox certainly is not central authority. If you stop being their customer, they will have just to live with that.
That's precisely the theory behind free governments the world over, to wit: "Governments are instituted among Men, deriving their just Powers from the Consent of the Governed". Except we all know in practice there are barriers to withdrawing consent from the government, just as there are surely barriers to leaving a market.