The deal with any 3rd party app is don't ask, don't tell. If it isn't mentioned in game as being used, they can't prove anyone is using it. So they are against the ToS but they're limited in actions that can be taken since they won't act on things outside the game.

Now if this person is killing mobs consistently deliberately to refuse others the chance, then you could submit a report on that behavior and see if anything happens. Because they would be able to tell in game at least how the person was intentionally griefing.