Wrongful billing after cancelation

I called Comcast to schedule cancelation of internet service in May because I was moving out on June 30.

On June 27 service stopped. 
I returned the equipment on the first week of July.

In September Comcast is still billing me monthly for a disconnected service.

I called Comcast and was told billing would be corrected.

A week later I get calls from Comcast collection billing August and September. 
I called again and was told it would be corrected.

A few days later more calls collecting.

Now a Comcast supervisor says that although I moved out and returned the equipment, someone has plugged equipment and I am liable for it. I asked him that if I am the only one with access to the account, how is it possible someone else paired equipment at that address when I don’t even live there anymore?

He say he will transfer me to a department that can fix it, instead he sends me to a recording that disconnects. I call again and now Comcast system identifies my number and will not allow me to talk to an agent anymore unless I pay all the months I did not receive service.