How do i turn off purchasing for an adult who is really a child?

Apple has ignored the case of family members who lack the requisite judgement to make credit purchases. This is our special needs population, people who may not be able to govern their impulse to buy things. The family group manager needs to be able to block any family member from making purchases. They should be the sole arbiter of this. There is no benefit to Apple's customers in allowing uncontrolled purchases. This situation is a flaw in the family sharing interface.