Sounds like you're in a no-lose situation.
First and foremost, I would certainly not let your pets eat the dog-food if you suspect it may be a knock-off.
Second, maybe as a courtesy, you could notify the company that you have canceled the credit card they have on file, and you are still willing to return the items if they end you a pre-paid shipping label. Tell them that, otherwise, you are going to discard the dog food, and will not respond to any further communications. (E.g. "last chance!")