How do I restrict Apple family member from making purchases?

The Apple Support articles I linked below offer several options for restricting purchases for minors. The articles offer step by step instructions for setting up the restrictions and modifying them to fit your particular needs.

Approve what kids buy with Ask to Buy - Apple Support

iPhone User Guide - Apple Support

Set up and use Apple Cash Family - Apple Support

Remove them from Apple Family/Apple Cash Family. Adults aren’t expected to be managed or restricted. If you have power of attorney, you could approach the banks about limiting access to funds.