^That was discussed at length in a previous suggestion topic and was never resolved.
to gerrard, if you summoned the MVP (be it with a prof or bloody branch) then take screenshots of anyone kill stealing it; make sure their name is visible. If it is a naturally spawned MVP then anyone can have a go at any time. The only MVP's on custom maps without PK are Hollowring and Skoll, so those are free for all. The reason players get away with breaking the rules is because the ratio of players reporting it is very small compared to how many offenses there are.
For better or for worse, the for_filds were designated newbie leveling grounds. We're a bit damned if we do damned if we don't; at the moment high wizards (mostly) take advantage of the fact that you can't kill them, and swoop down on your mob. But if we removed PVP, you know stronger players would take full advantage and go there to have some random fun at weaker player's expense.
What I can suggest is a compromise involving making for_fild02 PK again, since that is the map out of them all that is the most heavily KS'd. That also gives new players the chance to level on the other five maps, and also gives them the opportunity to defend their mobs.