Yeah, I mean that's basically how states form in the first place. The Ancap answer on how to avoid this would probably be (in theory) to have a population that wouldn't go along with it (this is part of how the Ancap -> fascist pipeline works, they end up wanting an ethnostate where they can kill leftists and other "undesirables"), or to simply assert "that wouldn't happen" (contrary to all of history). In practice, if an Ancap project devolved into that they would simply declare that it's not real Ancapism.

The ideology is extremely detached from reality and it's adherents believe in bizarre principles with absolute faith. Many in the cult explicitly reject the idea that historical or empirical evidence could falsify their beliefs. I think my favorite of their brain-meltingly stupid arguments is: "The fact that we're having this discussion instead of physically fighting proves that you already accepted the Non-Aggression Principle" which is the most thing I've ever heard in my life. They are, without exception, debate perverts.