I've never experienced this. Maybe I don't play games with people who get to that point so I don't see it but if someone did reach the point of being a 6 year old about a game then I probably wouldn't ant them there in the first place.
Some people do and will act like a child when they don't get their way. I for one, do not entertain any kind of immature or moronic behavior in my space. If you want to play with the sand in your own kind of way regardless of others around you then go get your own sand box!
I would let them leave. It would not be good to play with someone who would always want things to go his/her way just to win. Maybe he could invent his own game where he could make his own rules and where he could bend them so he could win.