Usually, if you write deceased on the envelope then mail it back the company will stop sending the mail. Or make copies of the death certificate to send to the company with a polite letter asking them to remove the name.
Write on the envelope deceased and no longer at that address. Then the post office will return it and they can update their records. Most places have no idea that a person is deceased they have no way of knowing unless a family member notifies them OR unless mail is returned with deceased written on it.