"[Voluntaryist] claims that private arbitration can replace all current [state] legal practices is wrong—there'd be no enforcement mechanisms for such arbitration!" (Daniel Punaniel)
This is a plain misunderstanding of the Non Aggression Principle, perhaps made by latching onto the name rather than doing the most basic research (like clicking a link or searching Wikipedia) into it. The non-agression principle is not pacifism; it is a little more complex than the short name (that's why it's a title and not the principle itself). It condemns the non-consensual initiation of aggression against peaceful individuals, but not every act of violence. ("Violence", "force", and "coercion" are fairly neutral; "aggression" is less so, so that's another clue, too.)
While "non-aggression" is in the title, it does not mean "pacifist"; the principle allows for self-defense, and even retribution for harm done.
Consider a boxing match—force, yes, but consensual.
Consider self-defense against a home invasion—force, yes, but defensive, not initiatory.
Consider force used to take back goods one party has in effect stolen (by accepting them via contract but not living up to their end of the deal)—force, yes, but defensive, to take back what was stolen.