It sounds like something that was sent through in error. Are you receiving more than one bill from Optus?
The Cable service must be showing as active as active on our sided, or your contact details simply haven't been removed from our distribution list. Potentially, we could have outdated information on our mailing list.
Best to get in touch with our billing team and confirm that the service is not still being billed for. You can chat to us over Messenger or send your details through to our Social Media service team. You can find them on Facebook or Twitter.