How to stop sharing iCloud storage plan with a family member

If you share iCloud storage with your family there is no option to exclude someone (but there is an option in Settings > Family Sharing > iCloud to stop sharing with the entire family).

When setting up iCloud storage sharing, anyone that is not paying for their own plan is automatically included in sharing; if they were paying for a plan larger than the free plan, they have the option to keep that plan or join the shared plan.