How do I cancel a merchant that automatically deducts monthly payments?

Have you checked your subscriptions?

Purchases - View, change, or cancel your subscriptions.      *

Purchases - Subscriptions not billed through Apple    *

Link to contact Apple Support at the end of the article.

Purchases - the subscription that you want to cancel is not listed    *

Please click on this link to learn "How to cancel a subscription from Apple" ➔ https://support.apple.com/HT202039

There are multiple other things it could be but you aren't very specific as to the nature of these charges. Is this something to do with Apple Pay?

Here are the two documents that cover two possible scenarios relating to your card:

If you see an Apple Services charge you don't recognize on your Apple Card - Apple Support

Dispute a charge to your Apple Card --> https://support.apple.com/HT212270

If this is an Apple subscription such as with an app on a mobile device, read the document I just linked. If it is something else we need more information such as where you are seeing this. I have no idea if you're asking about an app subscription, something to do with Apple Card, something to do with Apple Pay...?

Vern 716, I am so flattered to see you are being officially recommended in a repeat suggestion to do what I recommended you do 10 hours earlier, even if there is no evidence that what you are dealing with is a standard Apple subscription. I tried getting that information from you but you are not telling us what this is for other than a charge on an Apple Card which for all I know could be an old-fashioned newspaper subscription. I do tell you how to resolve Apple Card issues which is what this appears to be but that suggestion is apparently not relevant.