Probably a walled city is the only way to achieve that. Such a setup is like a closed mailing list in which all participants have to pass an 'entrance exam' of

Probably a walled city is the only way to achieve that. Such a setup is like a closed mailing list in which all participants have to pass an 'entrance exam' of some kind. This kind of setup imposes a cost for rude, nasty, aggressive and destructive behavior - since you can simply be excluded from the group in that case. This is how most of the world works on a day to day basis; you don't let random people into a corporate strategy meeting or an academic council meeting. You only admit people who agree to abide by civil society rules. Something like that can be incredibly useful, but it requires a lot of up-front organizing effort. Also, malicious types have been known to sneak into say, a closed mailing list, and leak the internal discussions to Twitter or whatever, so...