you could probably make a game where the game is completely won and you're just toying around with other countries/allowing them to exist, but the interaction that makes Civ interesting is most of the time not gonna last past a certain point. usually you just hit a mark somewhere in the game where you've got more territory, points, and production capacity than everyone else and at that point the game is basically won and you can do it however you want.
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http://www.youtube.com/watch?v=npHDxSvwCE0