I don't think that ArenaNet can do much about it, since according to their own rules someone "playing" a Necromamcer with Minions or an Engineer with Turrets while watching Netflix and pressing a key once in a while is not doing anything forbidden.

And how would you determine whether this someone, their trained cat or a macro is randomly producing key strokes?

While I personally think that massive amounts of AFK farmers are immersion breaking, there are far worse shenanigans going on, like botting in PvP. Also, if weekly updates require maps to be respawned freshly and players can actually prevent this by still playing (or farming) on some map instances, then the system is not well designed.