How can I use Family Sharing without being charged for family member’s app purchases?

I enabled family sharing for my girlfriend, so she has access to my Apple One subscription, giving her unlimited music and so on. But every time she purchases something through the App Store, I am charged for it. She is being very excessive with it, causing my bank account to go into the negative. I need a way to stop it without removing her from Family Sharing entirely, or buying gift cards (which is annoying). She needs to be responsible for her own purchases. What are my options?