user login form: name and password default values
Usually, it's the browser that remembers login names and passwords, allthough some sites place a cookie upon successful login that they use to populate the username field on your next visit.

I don't think Drupal does this by default so I think in your case, it's the browser that remembers the login name and password.

There is one way to avoid this and that's by changing the name of the form fields. Browsers 'know' to remember usernames because the name of those fields is usually something like 'user', 'username' or 'login'.

If you change the name attrribute of the login input field to, say, 'bycicle', browsers will stop remembering those values automatically.