Hi LoveAnother66,
Thank you for posting in Apple Support Communities. There is not currently a way to block specific merchants on your iPhone however if you are being charged for a subscription that is billed through Apple, you can cancel the subscription to keep from being charged again. Your bank may be able to help with stopping payments from the merchant, you would need to reach out to them directly.
Check out how to manage the subscriptions billed through Apple here: How to cancel a subscription from Apple.
If the subscription that you want to cancel is not listed
If you need to request a refund, check out this article: Request a refund for apps or content that you bought from Apple.
If you need further assistance, you may be able to reach out to the billing team at Apple Support for further assistance. Depending on your region, you may be able to reach them here: Get Support.
Take Care!