Most people in the Voluntaryist community tend to more or less agree that the only time violence is acceptable is in self defense. There are some exceptions to that, but they tend to revolve around personal morality and what a given individual would or would not do. However, as with everything else, there is some tendency to confuse what exactly that means.
What is defense?
Defense is an action taken to prevent bad thing X from being forcefully imposed upon you or someone else by a third party. What is and is not bad enough to warrant violence to prevent it is, of course, highly subjective, but there are several things that are widely accepted to fall within the realms of bad enough, namely, murder, rape, assault, theft, and vandalism. All of those actions are, to one extent or another, removing individual control over one's property.
(for those of you not in the know, you own your own body. That's right. It belongs to you, not them. A lot of people seem to have difficulty with that concept)
Now, here's the thing. Even within the anarchist/voluntaryist population, there is a split about what exactly that definition means. And, that split is centered around the immediacy of the threat against which you are concerned. Aka, threats that are in the past, present, or future.
Future Threats
As far as future, that's a tricky game to play. If someone says, "I'm going to kill you now", are you justified in taking violent action to prevent that from happening? It's an issue. Maybe they were making a tasteless joke and you didn't realize it, or perhaps they're simply an impulsive speaker and regret the words as soon as they finish the sentence. It's a tough choice, but really, it's not in the realm of 'the future', because they have voiced the threat in the moment. You have to make an assessment and determine how real the threat is, and act, or not, based on that assessment.
No one that I know of is saying that you should wait until you have a gun in your face before you take action. However, what if they said, "You just wait. Me and the guys are coming around later and you're gonna see the business end of a baseball bat."
This is firmly in the realm of the future. No immediate threat is present, and it's possible that no future threat exists either. Most people can agree that the appropriate course of action is to keep your personal defensive tools close to hand and see what happens 'later', and perhaps alert any associates that may be inclined to assist you in the event the threat is real. Few people would assert that this is the time to pull a gun and put one between the eyes of your possible future assailant.
Furthermore, and I can't believe I feel the need to include this here, you can't attack people just because you don't like their demographic or their lifestyle or they're 'ruining the neighborhood'. This should go without saying, but for some reason, it never seems to do so, especially with politicians.
Past Threats
Now, the thing about the past is that you have a lot more information about it than you do with possible future threats. For instance, you know what the other person did or didn't do, you have a much better idea of how effective their actions where, etc. This is where people get twitchy. Attacking people for something they did in the past is, in fact, not defense. It's reprisal. Punishment. Perhaps building a reputation for possible threatening individuals to reference in the future. But whatever has happened in the past is, quite firmly, in the past, and you are completely incapable of altering how it all went down, no matter what level of violence you bring to bear on the issue.(barring some kind of time travel option)
However, many people point to repeated violent aggressions in the past as a justification for taking preemptive violent action against the aggressive parties in order to prevent them from being violent in the future. This is simply punishment of future crime. The past does not predict the present or the future. Be wary around those persons, guard yourself against actions they may take. But initiating a violent action against someone who is not being violent in the moment is not defensive. It is offensive, and a hallmark of the violent institutions that I, for one, wish to bury firmly in the past.
Present Threats
This needs very little thought. If there is in imminent danger from someone, deal with it however is necessary for you and any other innocent people nearby to stay safe.
