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Not intending to sound flip, but you can quit by not playing anymore. You can't delete your city once it is created. It can be abandoned though, through your castle. Click on the Castle, you'll see a tab to abandon city. Click there. Though you could remove the application from your FB apps, but your city will remain there.

The above is not actually correct. If you have two cities, one seems to have the abandon on it but the other doesn't. Once you abandon one city, or only have one city, it's no longer an option to abandon.