I think a system similar to the one used in Neverwinter Nights would be a good idea, where you have certain areas, like wilderness which are fully pvp enabled, more where you can harm enemies and not friends, and some exclusive no pvp areas as safeholds like cities and outposts. Also that when a pickpocket wants to steal something from another player, they have to select that they want to dislike the player, which enables them to perform hostile actions, when this happens the target player can see in the player menu a dislike icon next to the pickpocket\'s name, but if the player is not wary enough to see this, they can be pped without warning.