You can completely remove a member from your account and profile (this is especially helpful if someone has left your organization and you don't remember which notebook he or she was involved in).
To do this, click on your avatar in the lower-left corner and then click Subscription.
If you are on the free plan, click View Advanced Plan Options, and then click Upgrade. With a Plus plan or higher, you can click on the box within the "Status" section for more information.
From here, click on "Members" to view the details. All users on your account will be listed, including notebooks they are a member or even an owner (highlighted in bold) of. To remove them completely, click/tap "Remove" next to the user's name.
Note: If the removed user is an owner of one of your Collections or Workspaces, you will receive an error message. Ask the person to be removed to transfer ownership to another team member.