If you don't mind setting up any other email addresses back into the mail app, you could always try clearing cache and data for the app. From your homescreen, press menu>settings>applications>manage applications, press the all tab at the top of the screen, scroll down to and press Mail, select clear cache, clear data. You will loose all data associated with the Mail app (email addresses, passwords, notification settings, etc., your emails will still be available on the associated website), but it will clear out the Yahoo account.
The Rezound has been notoriously bad with Yahoo email, but that will be fixed in a future update.
