To me its more like closing the bar they hang out in, almost all of them will find other bars to go to instead almost immediately but it'll cause at least a temporary fracture and even when they all do find themselves back to the same place (asides from the ones who just got on with their lives), it'll take a while to build up the same kind of feedback loop. Honestly, as a teen (over a decade ago now...), there were forums I was on that totally died due to a week or two of downtime, sometimes just breaking someone away from their familiar habits is enough to get them out of a bad routine.

The notion that we could reach them on 8chan is a bit idealistic, the only thing we're able to reach is the manifestos within seconds of the news breaking as far as I can see. If your goal is to _reach_ them, then do it in reality.