Strategy Page, one of the very best websites on military matters, explains why the pirates are immune from attack, and, under current conditions, will never be suppressed. (Further explanation here.) Ironically, while international law does not deter the pirates, it does deter their victims, and thus encourages piracy. Which is but a small example of how contemporary international law has been perverted into a tool against international commerce and civilization, whose protection was the very purpose of the creation of international law.
UPDATE: Wonderful essay by Mark Steyn on the same topic, noting, inter alia, the effect of U.K. government interpretation of the European Human Rights Act.