How do I stop Apple from charging my card when there is no money on it?

I want to stop Apple from letting me buy things when my card doesn't have enough money on it

No one can stop you from purchasing anything except yourself.

You can download free apps, but you may need to have a valid payment method on file, even if the app you want to download is free. 

You can have a credit card / debit card on file and an Apple Account balance.

If you have set your Apple Account balance as your default payment method, your default payment method will be used first by Apple when you need to make a payment.

Free apps has also In-app pruchase.

See -> Buy additional app features with in-app purchases and subscriptions

Buy additional app features with in-app purchases and subscriptions

See also -> Apple Media Services Terms and Conditions

Legal - Apple Media Services - Apple

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If you don't have money, don't buy anything.

If you don't have money, add money to your card or to your Apple Account balance.

See -> Add a payment method to your Apple ID

Add a payment method to your Apple ID – Apple Support (UK)

• If you want to change your card, add first the new card, then remove the old one.
• or if you want to use your Apple ID balance, you need to set your Apple ID balance as your default payment.

Or see -> Add money to your Apple Account balance

Add money to your Apple Account balance - Apple Support