Apple - Make iTunes stop prompting me for my username and password

All you need to do is Sign OUT of the itunes store!

If you are signed in, each time you fire up iTunes, it will try to login again and ask for your password. Sign out and it won't bother you!

Try to resetting your warnings, as suggested in this thread:

Open iTunes/preferences/advanced, then hit the box in the middle that says "reset warnings".