In order to ensure that a user is deleted from Microsoft Planner, you delete that user in the Office 365 admin center. Once the user is deleted, their telemetry information will be deleted in Microsoft Planner and their user name changed to "Former user". If you want to delete information other than user name and telemetry, you'll need to access the relevant plans and do this manually.
To delete a user from Planner
As the Office 365 admin, go to the Office 365 admin center and delete the user following the steps in Delete a user from your organization.
This will delete the user from Azure Active Directory as well. You have 30 days to restore the account before the user's data is permanently deleted.
Important: If you delete the user from Azure Active Directory first, you will not be able to export the user's plans from Microsoft Planner. Make sure to export the plans from Microsoft Planner first if you intend to also delete the user from your organization.
What gets deleted automatically
Once the user is deleted, their telemetry and user name are deleted, as follows:
All telemetry data is deleted. This happens every time a user is deleted from Azure Active Directory.
The user's name and User.Id are deleted. Microsoft Planner stores the user's name and User.Id against the TaskUserID. When a user is deleted, Microsoft Planner retains the TaskUserID, but deletes the user's name and User.Id. This enables Microsoft Planner to:
Rename the user to "Former user". All deleted users will be renamed "Former user".
Keep information on "task created by" , "task assigned to", and "task completed by" under the name "Former user".
To delete specific user data
Since Microsoft Planner is a collaborative tool in which much of the work is shared, deleting a user does not delete any plans, tasks, or content that the user was involved with or referenced by. If you want to remove or redact data included in plans and tasks, you can manually edit the plan.
To edit plans, do one of the following:
Grant yourself permissions to the group containing the user's personal data. This is most useful when the user's data is somewhere the user themself doesn't have access to.
Change the user's password and sign in as the user. This works best for plans that the user participated in.
Important: Changing the user's password and signing in as them only works if you do it before you delete the user from your organization.
Once you have permissions or are signed in as the user, you can change or remove any plan, task, or other content that should no longer reference the user.