After a user account is removed from the Administrators group in the Microsoft Office 365 portal, the user can use another browser instance to access the Office 365 portal, and the user still maintains his or her full administrator user rights. This behavior persists until the user signs out and then signs in again.
This is a known issue that exists in Microsoft Office 365. The best option to work around this issue is to confirm that the users are all offline before you try to remove the users' administrator user rights.
This issue occurs because the user must sign out and then sign in again to the Office 365 portal for the new user rights to take effect. And, any authenticated session remains active until the cookie expires.