i don't want to get overly weberian on you, but the state needs to be able to stop this. otherwise, there isn't a state. anarchists would deal with this by declaring war against and ostracizing the thugs, but this is the kind of social dynamic where anarchism breaks and the theorists start arguing that the society doesn't meet the preconditions and isn't ready. the basic hobbesian social contract is that the state needs to stop this.
the americans are beginning to signal that, if mexico doesn't want to stop this, they're going to finish the war they started, themselves and, on some level, they will be justified in doing that, if the mexican state refuses to exist.