As a pacifist, I oppose violent solutions to problems. This means I oppose war: war is always destruction, death, humanitarian disaster, and horror. I also avoid using violence in my own life (not that I am ever in a situation that requires it).
However, as a pacifist, I do accept force and the threat of force to maintain social order, safety, and freedom. I accept this force in the existence of the police.
Though I oppose the violence of war, I accept the force and threat of force the police use; I accept this for a few reasons that distinguish "force" from "violence" in my mind.
1. The police are trained to use force only when necessary, to use only the minimum amount of force necessary, and to use lethal force only when their or others' lives are threatened.
2. The police are generally able to use force in a controlled way that nearly eliminates danger to innocent people; this distinguishes police force from the violence of war. In war, there are always innocent people killed or wounded. Some call this "collateral damage," which is a euphamism for "innocent people murdered and property destroyed." Some degree of force is acceptable to me, but is always bad if innocent people must be harmed in the process. Small-scale police activity can manage to protect the innocent; large-scale warfare simply cannot.
3. If there were not some degree of force or the threat of force managed in a controlled way, then there would be a great deal of violence inflicted upon innocent people by those willing to use violence for their own ends. One could use the same argument to justify war, I suppose; however, as a practical matter, war usually fails to create order and safety for a nation. War usually leads innocent people to be harmed, in the short-run of war, and the long-run of the country after being devestated by war. There are many cases of war and occupation being at least partly responsible for political, tyrannical terrors inflicted upon people of a nation in the future. As a practical matter, war is bad policy; nonviolent solutions are better suited for conflicts between nations. But in the small-scale situations in society, this force doesn't necessarily lead to future terrors.
In this way, I believe a pacifist can own property and can support reasonable police activity. The controlled force of police activity is something entirely different than the chaotic, destructive, and massive violence of warfare.