Rage quits and reactions

What do people think should be done if anything when someone rage quits a game?

Got a younger player at one of my LGS’s who is becoming a bit toxic. He will rage quit and finally fully stormed out of the shop last week. He was made the other players gave me the win by choosing turns when I cast expropriate. I was trying to win and amp up my storm count (Kalamax). Upon the others making a bad call he packed up everything and left saying we were idiots.

I want to figure out the best response to him and this at any rate. I don’t want to discourage him too much. However, I am not going to hand him a free win. I will not show up with 99 forests and a ghalta