I think it's not really the cache you have to clear. It's the cookie file.

You can either "Delete Cookies" from browser options (which will delete all the cookie files), or if you don't know how to do that of if the browser settings are restricted (which they often are on public computers)...

Just go up to the top of the FlyerTalk page and click on "Preferences" and then "Delete all cookies set by by this forum!" and that should do the trick.