I don't think any semblance of "war", "boots on the ground" or even "nuke them into oblivion" will work. The natural state of an organisation like this is instability, and it feeds and grows on it while devouring the civilian areas around it.
I don't think that all would-be mass shooters have the nerve to murder close-range with a sharp weapon instead of spraying at a distance. I don't think it's the same for the perpetrator. The gun is a much bigger enabler to somewhat more impersonal violence.
Has anyone played Invisible Inc. by Klei Entertainment? Fun turn-based stealth with interesting agents, despite the short campaign.
I'm looking forward to Dishonored 2, but I'm a little confused by the supposed choice between playing as Emily or Corvo. It sounds like Emily is the main character this time around and Corvo is an option you can choose if you've got nostalgia for the older game. Or are they both available in one playthrough but at different times?
Probably a bit impersonal for a villainous act, but whenever a city/castle under my control in a Total War game got too uppity (a metric of discontent), I vacated it, let them rebel, looted everything and left the broken remains to an enemy faction.
After the enemy faction kindly rebuilds the city, I take it back.