They disappear like a fart in the wind.

Escape? Then you need to place more armies around the city.

Burn them all to death. It's the only way they'll learn.

Well, supposedly the land carries unseen population who join your cities over time creating population growth. Therefore if you raze a city, those people return to the 'unseen multitudes'.

I doubt there is any 'actual' effect in the game that would result in increased population growth for nearby cities, so what Lord Xia says is correct.

... This can't be a serious question 

Well they certainly won't repeat their mistakes.. the main objective of teaching someone a lesson..

They get 3rd degree burns, their skin turns purple, and they can no longer eat dairy.

Haha, Shawshank.  Love that movie.