Still being charged after canceling the service

I cancle the internet server on August. However, I didn't know Xfinity doesn't cancle my mobie Xfinity. I call them September 27 to cancle the mobile account. At that time the agent say I don't need  to pay anything. Today, they charge me again. This time the agend said the bill for August to September 27. I don't have confidence the xfinity will cancle my mobie service. I ask him to remove my credit card so they don't auto charge in the future but he asked me to contact my bank to block xfinity charge. Please help provide document that my account is cancel and I want to dispute the bill