How do I get people to stop typing the state into the city field?

I have a form with city, state, country fields. The design makes the user select the country, then state, and then if the item is not found, they type the name of the organization followed by city. Roughly 5% of people type "City Name, ST". e.g. "Boulder, CO" or "Boulder, Colorado". There are cases where a comma makes sense, such as "Brooklyn, New York".

Is there any good solution to this problem?

Edit: We have 30% international addresses, so zip code tricks don't really work.

Update: I like Keno's idea of validating post comma text... show a soft error when there is a comma.