How can I stop a merchant from charging my account

You might want to check if you have scheduled payments to the merchant:

View your Apple Card balance and scheduled payments online – Apple Support

In Apple Support's article (link below):

Scroll to the bottom of: "Set up scheduled payments for Apple Card"

for info. on how to cancel scheduled payments.

How to make Apple Card payments - Apple Support

If needed: How to: Dispute a charge to your Apple Card - Apple Support

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or

Try Apple Card Support

(There is a "Get Support" link (online chat, phone, eMail etc) at the bottom of their website (link below)

Apple Card - Official Apple Support