Hmm ... then I think I need to be set straight on this. In the good-old-days "spamming" meant advertising. -- or otherwise I would have thought in this context, "non-game-related" communications.
But it seems to me that a public forum would have a real game-related function. (In fact, I am not TOO crazy about the idea that it might be a place to go and watch people "joke around.")
Let me give an example of how I have used it -- which to me feel legit, but which by your definition, it would not be. I had made a very minor stab against another player for which I made amends (or at least thought I had) but then I discovered that he was going around to other players in the game, telling them that I had "broken a treaty" and must be made to pay, blah, blah, blah.
Since I had no way of knowing how many people he had communicated this inflated version of events to, I decided to use the public chat to declare: "Listen, everyone, I am SORRY I STOLE SHASTA from Pink. That was wrong, but I made amends, and he knows it, and if he tells you I 'broke a treaty,' that is false." (I don't recall my exact choice of words, but it was not profane, and it was direct, but only confrontational in the sense that I intended to deflate the story that was apparently being passed around.)
This had the (admittedly) desired effect of causing Pink to fly off the handle in a righteous rage and then spend the next several days wasting resources by hurling himself at my prepared positions. I also think that it lost him the allies that he had had. It was a move that really turned the game around for me, and sorry, but it feels like the perfect use of a public forum. (Much better than joking around.) ... kind of like a United Nations ... type ... thing.
But of course rules are rules. Let me know if I should not be doing that in the future. Or if it should be just stuff like ... "hey" and ... "tsup"