By having a system of checks and balances in place, that allows due process and impartial review. In the US we have the judicial process in place for this. As an interesting note, SOPA proposes doing away with much of this process in the name of "streamlining."

Yes. And that's what puts vigilantes in a morally dangerous position; by definition they're not answerable to anyone. Or at least not answerable to anything more specific than the law and public opinion.