i think that a good way of handling the porblem could be the creation of police npc..
some strong pack of policemen patrolling cities will make someone think two times before attacking other people, just like in real life. Those policemen would wander patrolling and attaking or taking measures against unauthorized combats. this system will give a great improvement to our fantasy world politics and social relations, as policeman will certanly not be present everywhere and evil chars will gather outside cities in forests or whatever to pick on travellers. Reacting to this, people will travel in packs, hire guards, create guilds of paid protection and whatever..when a route becomes too hard to journey across, city governors could decide to send their men cleaning the area from crime and so on, creating a deep level of complexity and possibilities