Actually, from my experience, just having a moderator who weighs in occasionally keeps the tone a bit higher with no loss of freedom of expression. I moderated mailing lists for an addiction group and for a company and I never had to stop any piece of mail and only had to ban one contributor (who I now understand to have been trolling). Every once in awhile, I'd send a piece of mail (I did not otherwise participate in either list) sort of tooting the lifeguard whistle and things would calm right down.
Tom Parmenter Ortolan88