I personally think that with all these "Guard Guilds" around, why not just leave it to the players to guard the cities, roads, and other place where people could get attacked. If anyone is attacked and there isn't a guard nearby, then he or she can shout for help. People who have been PK'ing a lot can be added to a wanted list.

Think of it this way, in real life, just because you can attack someone without challenging him doesn't mean you would attack him.

Another thing to note is that the most experienced and most powerful players are the ones that would help protect weaker players who are attacked by lunatic newbies on a PK'ing spree.

Of course, it would really help if the combat was more balanced so that a few newbies would be able to take on an experienced player.