As a suggestion
Perhaps a paragraph under the Welcome All message, along the lines of
The site owner, reserves the right to remove anyone from the site, whose behaviour over a sustained period of time, in posting, is deleterious to the overall good of the site. It is expected this will be a rare occurrence
Basically trolling, which as you say Curious, does not breach the principle of free speech.
And I'm not talking about the occasional shit stirring comment, but sustained, ongoing behaviour
Trolling
Trolling – (verb), as it relates to internet, is the deliberate act, (by a Troll – noun or adjective), of making random unsolicited and/or controversial comments on various internet forums with the intent to provoke an emotional knee jerk reaction from unsuspecting readers to engage in a fight or argument
Describe anyone we know