block family members

Assuming that you are asking how to stop them purchasing things, then depending upon how old the family members are you may be able to enable 'ask to buy' for them : Request and make purchases with Ask to Buy - Apple Support

If they are too old you will either have to trust them, or remove them from the family. If they have a balance on their own account then that should automatically be used for their purchases instead of the organiser's credit card : Family purchases and payments - Apple Support