I tend to think that ther person doing the MPK is the problem, not the mob. The mobs are under their programme mechanics and they aggro wherever they are has important contribution to the thrill of this game.
My suggestions are:
Establish the crime scene evidence
(a) MPK usually happens in specific areas under a definite condition of HNM or NM spawned. SE may develop a log programme to record activities during these moments without much difficulties. MPK usually results in a lot of mobs trained and a lot of players KOed. They are conditions to look for upon reviewing the log. I dont think its that difficult to do so. The GMs have been around for so long, I dont think they can monitor that many locations plus they need to response to player's call for help from time to time.
Make the punishment to the MPKer painful and unforgetful
(b) the jailing system of restraining a player from playing for a few days are not strong enough. Besides, its known gil sellers played and handled multi-accounts by multi-ppl, of course. With strong evidence from log, the punishment to the MPKer can be in the range:
(1) striped of the most valuable weapon or armor in his bags, storage and mog house O.O);; ; and/or
(2) pay a fine equal to certain percentage of/or all the gils at hand; and/or
(3) striped of all but 3 most valuable armors in his bags, storage and mog house; and/or
(4) convicted offender is down lvl by 5; and/or
(5) including any of the above, but spread to his whole user account, i.e. striped of the most valuable weapon or armor in his or his mules bags, storages and mog houses; and so on
(6) repeated offender should be banned from the game. SE has not publicized its serious action against wrong doers for a long time =P .
Spread the Action to the MPK Core
(c) All Linkshells (as MPK LS may have multiple LSs to shift around) hosting the offender(s) including their members will be forfeited of getting any drops from NM, HNM for a certain period of time. Everytime a LS member logs into equiping this LS will receive a system generated message.
(d) Player checking on their LS will automatically received the system generated warning on undesirable behaviour of this LS. This will be on for a longer period of time than (c).
My 2 cents, not to start a flame war.