Only person one has to really respond to when out gathering is a GM if contacted by one.
If I was out farming mobs and got a tell from someone like that and they persisted or took it to shout chat, it would be very likely I would fill out one on harassment on them. Gathering without talking to people throwing accusations around is not against the ToS, but harassing/griefing others is. Might want to keep that in mind before trying to deal with suspected botters, as they can flip the tables on you.
I report them all the time when I see them, more often than not it is teleporting gatherers, or ones standing still killing stuff as it gets in range that are going to be the likely botters.