The family administrator can remove members from the family subscription. 

To do this, open family settings in the web application.
Press "Edit" to the right of the persons name. Confirm that you want to remove the person, in the pop-up dialog.

When you remove a member from your family, all the files on their user account will stay saved, and the user will be given an Unlimited subscription that expires in 10 days.

In the 10 day period, the user can cancel his subscription, or add payment details to continue with an unlimited subscription.

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