Note: This only applies to the Thanx app, not Thanx-powered branded apps.
To remove a merchant card from the app, find the merchant you want to remove and tap or click it to see the merchant details.
Click on "More" and scroll down you'll see "Favorite merchant" and toggle it.
Scroll to the bottom of the screen, and you will see a toggle labeled "Favorite Merchant."
If you click this it will remove the card. Note that if you make another purchase there it'll return to your list of favorite merchants.