I believe anytime you make it clear that you are uncomfortable with someone entering your personal space and they continue to advance to your guard, you are justified in a pre-emptive strike. Also, things like the eyes opening and then narrowing, tension in the upper torso, clench fists, etc make it clear that they intend to do you harm. I usually say very loudly, "You are making me uncomfortable; please don't come any closer." Then I ask them what they want. Then I strike and continue striking until they are no longer a threat. If there are alot of people around I might yell things like: "I told you I don't want any trouble." Of course, I am elbowing, kneeing, and striking them as I yell. No one has the right to try an intimidate you by entering your personal space.