how do I decline a purchase I didn’t make

Various possibilities including you are responsible for a purchase another family member made, and there isn't really any purchase at all and you are being scammed. Of course there could be a purchase...

If you see ‘itunes.com/bill‘ or an unfamiliar charge on your bank, credit card, or debit statement - Does not mention purchases made under Family Sharing which might be charged to the organizer's card but will not appear under the organizer's purchase history or subscriptions. Ask family members about those or check your receipts. --> https://support.apple.com/HT201382

Request a refund for an App Store or iTunes Store purchase - "You can't request refunds for recurring charges" --> https://support.apple.com/HT204084

Report A Problem to "Sign In for help with iTunes purchases made in the last 90 days" or ask for a refund --> http://reportaproblem.apple.com

Subscriptions aren't managed through Report A Problem , see:

View, change, or cancel your subscriptions --> https://support.apple.com/HT202039

If you have unsatisfactory results with Report A Problem or still want to try to request a refund for a subscription, contact:

iTunes [Store] Support - Use this form for assistance with account and billing, code redemption, or purchase questions --> https://www.apple.com/emea/support/itunes/contact.html

or

Worldwide iTunes support --> https://support.apple.com/country-selector/itunes