It's a nice theory but in practice it doesn't work. Most forum members can't or won't find or use the ignore function and at the end of the day trolls drive members away. Trolls attract more trolls.
Most forum members avoid confrontation and conflict on forums - they are willing to disagree but want polite differences of opinions. Trolls amp up hostility enough so that engaging them is not worth while for most folks - they just stop logging in. Trolls can kill a site in my experience.