If you no longer need a city, you can delete it.
Choose View > Cities & Time Zones.
Select one or more cities in the list.
To select cities that are next to each other (contiguous) in the list, press the Shift key and then click the first and last city; to select cities that are not next to each other in the list, press the Command key and then click each city.
Click the Remove (-) button at the bottom of the list or press the Delete key. Cities that are associated to clocks can't be deleted.