You won't be able to stop the alerts until you update your payment method. You must verify the existing one or add a new verifiable one.
If you did not block the card from your bank's side, then the payment would have gone through directly as normal and your card would not have stopped being verified from Apple's side.
At this stage you must verify the existing card, or add a new one that can be verified.
Again, charges cannot happen randomly or by mistake by the merchants. Subscriptions to apps are handled direclty by Apple, and only charge you if you started them. If you downlaoded a free app, that had a free trial, those will in most cases renew into paid subscriptions which will charge you. The Merchant doesn't have any real involvment in those charges.
If you failed to cancel it, then it charged you. It went through and you complained, but clearly something esle happened to cause the card to no longer be verified with Apple. Normally this should only happen if you blocked the charge from your bank or card issuer's side. Apple will not just randomly cause a card to stop being verified.
Add a new card to verify the current one to stop those alerts.
click here ➜ If you need to change or update your Apple ID payment method - Apple Support