Q: Turn off via windows

See Change or remove your Apple ID payment information - Apple Support.

Do you mean home sharing or family sharing? I can't see why you cannot change or remove your payment information in the first instance, but it makes a certain kind of sense in the second.

tt2

What happens when you attempt to turn it off? Are you the person in charge of the Family Sharing account?

Hmm, says here: Family purchases and payments - Apple Support

If you need to, you can change the billing method on file in your account at any time in your iTunes & App Stores account settings.

I suspect you can change to a new valid card, but cannot remove all payment methods while the family sharing account is active. After all if you remove your payment method how will the other family member's purchases be paid for? To remove your payment method completely I think you would need to disband the family sharing account altogether. See Leave Family Sharing - Apple Support.

tt2

Now I understand. You don't have any iOS devices or OS X computers and you want to disband your family sharing account. You'll need to contact iTunes Store support using the link at bottom of the page here: Leave Family Sharing - Apple Support.

tt2