Joe Nusz of The Debate Files has written a few articles on the vigilantism resolution, analyzing it from a couple different ethical perspectives. (Each of his links below points to a longer piece in PDF form.)
To Nusz's take on the Big Three Social Contract positions (Hobbes, Locke, Rousseau), I would add that Hobbes' position, though strongest for the Negative, also puts the Neg in the bind of defending a form of sovereignty that contemporary folk find unappealing. Thus, Locke is probably the best option for the Neg, since his theory of the contract is less autocratic, yet his condemnation of vigilantism is unequivocal.
Nusz also introduces the philosophy of Bernard Williams (for advanced debaters, I'd say), and discusses good ol' Immanuel Kant. Whether you're looking for a new direction or a classic approach, there's something there for you.