To stop your client from automatically login into LiveJournal, you'll need to open up a new journal entry dialog box. Click on the LiveJournal menu (top left corner) and logout. This will give your client login dialog box. Uncheck the "auto-login" feature and log back in to accept the new settings. You can log out and close the client. Next time you restart the client, it will ask you to click on the login button.
If you want to remove the saved password feature, follow the same procedures as stopping the "auto-login" feature. This time uncheck the "saved-password". Log back in to accept the new settings, log out and client client. When you reactivate the client, it will no longer have the saved passwrd.