[Lazortag]

I think you absolutely should snare griefers while they're engaged on NPC's. The only way to learn empathy is for them to understand the impacts of their own behaviour. After killing them this way, you should have them sing the empathy song for added effect:

"Empathy, empathy,
Put yourself in the place of me!"

Also if we're on the subject of creative ways to grief people, I think mez-pushing someone into Kithicor (while it's down, so pretty much all the time) is the best.