You can't erase your city, it will stay on map as long as Plarium wants it to stay.

If you don't want anyone to farm your town you need to demolish all resource buildings and kill all troops.

You cannot erase your city, its how Plariun can advise that they have millions of players playing the game, but only a few hunderd thousand of tehm is active......

Best thing to do is destroy all your buildings then unlink the game from your google play / facebook. Then delete the app.