One group I manage -- the official message board of a 501(c)(3) -- was set to all-moderated almost twenty years ago due to posters failing to trim replies. We quickly gave up trying to appeal to them. It just doesn't work. Now everyone is moderated and we trim their messages for them.
Lately we have a new one. Members replying to a thread with a message about another, second (and unrelated) thread. Example would be:
Subject: Re: GDP drops in third quarter.
Text - I had that same problem ordering airline tickets in advance.
But it's a moderated group. We just edit the subject -- . Subject: Re: Best time to make airline reservations -- and approve the message.