Knowledgebase:
Chromebook - Logon Credentials Being Remembered
Posted by Respondus Support
If you have Chromebooks being used by multiple students, and student logon credentials are being remembered by LockDown Browser (even after shutting down the app and restarting the Chromebook), it may be due to a setting in your Google Admin Suite and/or the SSO you are using.

LockDown Browser does not cache logon credentials.

Our testing shows that similar behavior occurs outside of our app when using a Google SSO and the Google Chrome browser. Testing results indicate that the Google SSO is not properly invalidating the logon credentials once the browser session ends. 
We recommend that you investigate how your SSO can properly invalidate these credentials.

There is a way to delete all user credentials via a setting in the Google Admin Suite. In the Admin Suite navigate to "device settings/sign-in settings."  You will likely see an option for "User Data."  If "Do not erase all local user data" is set, you can optionally change the setting to "Erase all local user data”. But please be aware that this setting will require your students to re-enter your institution's name each time they start the LockDown Browser app, and also could delete their user account from the device. It's similar to performing a reset on the device.

In light of this, we think the better course of action is to pursue a solution via a setting in the SSO.