How can I invalidate the created discount codes?
If you configured discounts in the Frequently Bought Together app, a new discount code will be created every time a bundle is added to the shopping cart using the app.
Although unlikely, it is possible that a customer kept a discount code and tried to apply it again after you disabled the discount feature or uninstalled the app.
Please note that the discount code is only valid for the particular combination of products that was created for and that it can only be used once per customer.
However, if you want to be completely sure that the discount codes cannot be applied in the future, you can invalidate them from the Shopify admin:
You can invalidate all discount codes in bulk. See the Shopify Tutorial at https://help.shopify.com/manual/promoting-marketing/discount-codes/discounts-faq#can-i-disable-multiple-discounts-at-once for more details.