Hmm, might have the wrong idea here. The person in no way is a wiz. They dont even need a special title if thats whats worrying you, someone showing higher than HM on who.

Plus they have no punitive powers other than if you intentionally disrupt a quest to be spiteful then of course your practicing harassment.

Regarding killing them, you can as always. If they started a quest while being hunted that would be no different than other abuse. Now if you suddenly just have the urge to kill them while their running a quest, sorry, dynamic quests for players take precedence, you can kill them after the quest.

A side note would also be they could have a second character who joins no guild whose sole purpose is to log in with when they have a quest they want to run, whether its a snatch and grab or whatever.