Won't stop sending me bills after account is closed (no balance due)

I ended my service with Cox in June (moved somewhere they don't offer service) and I am still getting monthly bills through email and text. I've contacted Cox four times and they keep telling me "your account is closed and there is no balance due, I don't know why you're still getting bills, just ignore them." No one at Cox is able to help or willing to direct me to someone who can. How do I get Cox to stop sending me bogus bills? It's frustrating and making me paranoid that I still owe them money and am going to get bad marks on my credit, even though they insist there is no balance.