Maybe rules can be enforeced by a grand observer NPC, they can\'t be killed and observe a wide area.
A day is 24 hours, they take two 1 hour breaks a day, there by allowing for a thief/bandit/Pker to kill freely without being caught. Course to keep the risk up, the break times are entirely random and determined at the begining of the day. People who don\'t PvP will need some sort of protection... maybe those can see marked PKers after they visit a specific building (a police station like area where a list of killers are listed).
If you are caught by the observe to be PKing then you will automatically appear in the wanted list with an incremental bounty depending on how much you kill and how powerful a person tou have killed, this lets bounty hunters have fun and a valid job.
To not be marked by the observer as a PKer you must challenge that player for a formal duel, in such an event, neither side will be marked for PKing.
A rule should be placed so that it\'s the person who initiates the attack against another player first that ends up on the wanted poster. After all... it would suck if a merchant became wanted just because he killed a bandit in self defense.
The marker on a PKer can either dissapear after a set time (maybe 3 days to a week?) or assuming a set bounty deduction rate, when the bounty hits 0.
EDIT: PS. I have never played an MMORPG in full before, so I am making all this up and I don\'t know how well it would work... I tried to reason it out as best I could.