So it's time to tackle a weighty question: is property damage, strictly speaking, violence? Anarchists and assorted radicals tend to say with Proudhon that property is theft, and that property owners deserve what's coming to them. Rich people and centrists tend to say that property damage is as bad as physical assault, and that vandals should be punished harshly. Despite my anarchist leanings, I don't fully agree with my faithful straw anarchists - I find it hard to draw a strong distinction between property and not-property that has my body on one side and my possessions on the other.
Can we say that since property damage causes emotional harm and physical violence causes physical harm, physical violence is worse? I don't think so. Certainly there are kinds of property damage that would cause me physical harm, and kinds of physical harm that would cause me no emotional harm. I've been working on a novel manuscript for the past year. If you destroyed that and all copies, I would consider that you had attacked me - the act of deleting files and burning paper would constitute real physical harm. If, on the other hand, you held me down and cut my fingernails - undeniably part of my body - I would think you were strange, but it wouldn't bother me if I never felt I was in actual danger.
What I am saying is that all harm is property damage. If you consider your body to be your property, then somebody who harms it damages your property. To the extent that you consider an object in your possession to be your property - very much so in the case of my novel manuscript, not much at all in the case of my fingernails - damaging it harms you. A Stoic whose only property is his ability to make moral judgments can only be harmed by that ability being damaged. Beat him, burn him, lock him up and take his money, and he is unperturbed. The only person who can damage his property is himself. A rich man who owns much and considers his houses and cars to be part of his identity is heartbroken at the slightest property damage.
So if you break a shop window, are you committing an act of violence on the shop-owner? Well, maybe not, if the shop-owner doesn't care much about his shop. But this is a terrible excuse. If you punch a random stranger in the chest, that may not be an act of violence if the stranger turns out to be Stoic or Buddhist. That doesn't mean you're justified in doing so - since most people care about their bodies and shops, you are ethically obliged to assume that any person chosen at random will. Protesters who spray-paint buildings and loot convenience stores are not engaged in peaceful protest; they are engaged in violent protest.
Of course, this doesn't necessarily mean that property damage is wrong. Violence in certain cases might be fully justified, such as violence in self-defense or violence against an immediate oppressor. Let's own up about our violent tactics, though, and not use mealy-mouthed arguments to try and make ourselves seem blameless at the very moment the brick hits the glass.