Family sharing

If the account's age is set to less than 13 it cannot be removed from a Family Group, only transferred to a new Family Group.

If the account is over 13 it can be removed from the Family Group. Follow the steps here: Leave Family Sharing - Apple Support

Beyond that, accounts never stop functioning. If for some reason the email address used for the account is no longer functional, then its best to just update that. Otherwise, all purchases made directly by the Apple ID in question will no longer be available through family sharing once you remove it from the family group.

If necessary, there is a contact link.

Apple ID Support

Apple ID Support (2) 

Apple Support by e-mail or chat    Work through the screens to get a chat option.    

Apple Support using Twitter

Or fill out and submit this form.          The form is for iTunes, but it will get passed to the ID Support section.