Many anarchists use violence. If not against people, then against property, especially in protests. The motivating principle of anarchism, though, seems to be a rejection of coercion. To use Professor Crispy's definition (again), anarchism is the idea that all human associations should be voluntary. When you punch a cop, though, or recommend that crime victims hunt down and hurt their attackers, are you still expressing an anarchist idea?
Don't get me wrong. I think that, in certain situations, vigilante justice or punching cops (not necessarily mutually exclusive activities, mind you) are morally justified - but while I can reconcile those actions with my conscience, I have more difficulty reconciling them with my anarchist principles. Does coercion somehow become not-coercion when you're coercing somebody who's coerced you in the past?
In practice, anarchism comes welded with a libertarian-ish ideal of justice and retribution. Unlike libertarianism, however, anarchism is fundamentally idealistic about human nature and free will. It's also more principle-based than consequence-based, in my opinion: anarchists are more likely than most to say "screw it, let the world burn so long as I have my freedom". Again, I'm not condemning this attitude! Like Hume almost said, it's certainly not irrational to prefer even the total destruction of the universe over the slightest encroachment on your liberty. What I would like to question is how the idealism of anarchism works together with the idea of retributive justice. Because if it's okay to kill a man who murdered your friend, then surely it's okay to pay another man to kill the murderer. And surely it's okay to pay a group of men to kill murderers in general. Do you see where I'm going with this?