Make iTunes stop prompting me for my username and password

On OSX Lion, iTunes keeps prompting me to enter my username and password to "sign in to enable this computer for automatic downloads." In the past I tried checking the box to remember my password but it kept forgetting, so now I just disabled the "always check for available downloads" option in Prefs -> Store. It still prompts me. It drives me crazy. Is there a way to get it to stop prompting for my password, so it will just behave like a well-behaved offline music library manager, and only ask me for my username and password when I open the iTunes store?