It is essentially the non-aggression principle which defines whether or not someone is a libertarian. Adopt and advocate it, and you are one, otherwise not. In theory this could apply to anarchists also, though many would prefer to be called socialists before being caught dead thought of as libertarians (for whatever reason).

The bit about Somali law is interesting. Some have advocated the notion that the only valid libertarian law is the law of torts. If a person isn't party to the transaction in question, he should have no standing to sue. This may or many not explain or otherwise be connected to the fact that when you see a criminal indictment (as opposed to a civil motion for suit), the state is always the plaintiff. This sets up enormous distortions in that the interests of the plaintiff's counsel (state's attorney) are not necessary in parallel with those of the victim, the victim's heirs or with the interests of society at large. Justice would be better served by several different measures if state's attorneys were replaced by people who represent actual victims, rather than law books.

A version of the NAP could even be added to statist constitutions: No cause of action where a state or states are the only complainant shall be heard by any court. This of course would lead to the rapid collapse of the system and the discarding of the constitution, presuming that taxpayers would rapidly de-fund the state knowing that it had lost its extortion enforcement mechanism.