I belonged to a forum that an acquaintance (I'll call him Fred) had started after a forum that he and I belonged to had discontinued the "area of very specific interest" subforum that several of us frequented.
Shortly after Fred started this forum, four young women, who were friends of his (two of them were also his neighbors, and they were sisters) joined. Three of them were perfectly lovely, but the fourth (younger neighbor sister, I'll call her CG -- for Crazy Girl) had some serious issues with histrionics and anger. Sort of a cross between Veruca Salt from "Willy Wonka" and the "South Park" portrayal of Paris Hilton. I don't give a rip about football, but I was *praying* for CG's team to win the Super Bowl, because I knew she was going to go bat-poo crazy on the forum if they lost. (They did. And she did.)
I also think CG had a crush on Fred. She always called him "Fred Hun" -- somehow it never registered on her that "hon" is short for "honey" and has an O in it.
So Fred's forum had a free-for-all, anything-goes subforum similar to the Oubliette in the OP's post. Which CG proceeded to turn into her own private PM-to-Fred area. I remember one memorable exchange in which she was doing the text equivalent of *screaming* at poor "Fred Hun" that he *had* to come over and install a new OS on her computer RIGHT! NOW! The computer wasn't malfunctioning or anything; she just got it into her head that she had to have the new OS because it was new.
I told her, gently, that she really needed to take this to PM, that this exchange was painful to read, and the rest of us didn't need to be privy to it. She got very indignant because after all, this was the "anything goes" area.
Fred traveled a lot, and this was before the days of widespread WiFi, BlackBerrys, Internet phones and such, so he had to be home on his computer to access the forum. It took a lot of pleading with him (privately) by the rest of us before he finally suspended CG. I don't think he ever banned her, but she did clean up her act ... a little. Shortly afterward, the forum was shut down because it had been hijacked by a troll -- unrelated thing not involving CG.
The moral of this novel is that even a free-for-all subforum needs rules. In the OP's situation, I'd get with the admin and the other mods and agree to a set of rules. Maybe something like this:
Personal attacks are not allowed. Venting about the effing a-hole who cut you off in traffic is OK. Calling another poster an effing a-hole is not.
Slurs of any kind -- racial, ethnic, religious, sexual, sexual orientation-related -- are not allowed, whether the person being referred to is another poster or a third party. Neither is implying that someone's bad behavior is related to his or her ethnicity, gender, orientation, etc. So while venting about the effing a-hole who cut you off is OK, venting about the effing N-word C-word who cut you off is not.
One-on-one communication with other members should be done via PM if it is not something of general interest to the membership at large. "Fred, congratulations on the promotion!" is OK. So is "Fred, your new avatar is awesome!" But "Fred, want to have lunch at Chez Yummy tomorrow?" or "Fred, we need to talk about X" belongs in a PM.