It's not spelled out, as you already noted. On the face of it, it almost seems like something that should even need to be stated. And if I had to guess, I'd say the event that prompted this inquiry is from a certain few posts that were made in AR to communicate with a member that can only post in AR right now. I don't think that should be happening, and the person those posts were directed at doesn't seem like they intend to take action on those suggestions.
If that was happening outside of the lower boards, and/or if it was directed at a newer member or perhaps someone that just wouldn't understand that following such a suggestion would only land them in hot water, I'd be more inclined to take action. Unless and until the rules are updated to include something to the effect of "Instructing other members to break forum rules is forbidden," I think it's got to be handled on a case-by-case basis.