Square Enix acknowledges that a good MMORPG will have both positive and negative experiences with other players, but that the MPK form of harassment is excessive and harmful to gameplay. Therefore, in the next version update, two changes to the game will be introduced.
The first change causes monsters to disappear under certain conditions. A monster will normally wander around a set area when it is not attacking a player. Under the new conditions, when a player is defeated by a monster outside of that area, or if the player being chased leaves the area, the monster will disappear. This means that the normal method of MPK can't be completed on any player outside of a monster's normal range. As soon as the attacker leaves the area, the threat is neutralized.
The second change affects monsters that have been immobilized with the "Bind" spell. With the new version, a 'bound' monster will only attack the party or alliance that it is targeting.
While Square-Enix have implemented other methods to try and stop MPK in the past, it's thought that these are the most likey to prevent it from happening.
[Source: Galbadiax]