Dr. Block can certainly consider the word anarchy to mean "unjustified rule." However, I do not believe we can take the roots of words and necessarily derive what "the" current correct meaning of a word is.
For example, the word football is considered to mean soccer in many parts of the world. If we just take the roots of words, this would appear to be a more "correct" meaning for football since the foot is used almost exclusively in soccer to advance the ball, whereas it has only limited application in American football.
The bigger point is that words evolve overtime and can have very different meanings from their roots. This should be recognized.
In fact, just looking at roots can get us into all kinds of trouble. The root word for orchid comes from the Greek orchis, which literally translates as “testicle.” Even plain vanilla words can get us into trouble, and I do me vanilla. Vanilla is derived from the Latin word vagina. The word pencil is derived from the Latin word penis. I could go on but I don't want to conduct a seminar on etymology. But, speaking of seminars, the word seminar comes from the Latin semen, so does that mean only males should deliver seminars and only females should be the recipient students?
That said, anarchy is primarily considered by the general public to mean chaos and disorder, not unjustified rule (which btw I think is a distortion of the root word archy, which means rule, not "unjustified rule"). I believe anarcho-capitalists are building themselves an unnecessarily hurdle in advancing their views by referring to their desire for anarchy. It confuses most people and, quite frankly, makes the advocate sound nuts, since most will think he means he is in favor of chaos and disorder.
If libertarians are attempting to get their views across to the masses, it makes sense to use terms in the way they are generally understood, rather than use a word in a manner that creates immediate knee-jerk negative reaction for no good reason.
Indeed, that is why in the essay I wrote which is linked to in the bibliography Professor Block provides as part of his answer to Tim, I advance the term, Private Property Society, rather than anarchy. To me PPS conveys exactly the type of society I would like to see, without creating the possibility that someone will misinterpret me and think I am in favor of chaos and disorder.