Hi Peter,
The user account will be remembered if the user checked "Keep me signed in" on when he sign in to the Office 365 sites. The users' account information will be remembered in the browser's cookies.
To clear the users' accounts, please clear the cookies in your browser. For example, in IE, click Tools > General, click Delete under the Browsing history session, check Cookies and then click Delete.
Thanks,
Nancy Xie