Originally Posted by Chuck S
They need to clear their cookies
I was afraid that was the answer. Not a good situation, telling users to clear their cookies is like telling them to learn PHP and write their own classifieds, it won't happen. And even those that know how to clear cookies won't like to do it, since clearing all cookies will mess up their logins and "remember me's" to a lot of sites. And expecting people to find and clear only the zip-code cookie isn't practical.
I've added a request for a link to clear the zip code to the suggestions forum. Please consider it.