The family administrator can remove members from the Home subscription.
To do this, find the 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 Personal subscription that expires in 10 days.
In the 10 day period, the user can cancel his subscription, or add payment details to continue with a Personal subscription.