The worst experience ever

I had an issue over a year ago in relation to the incorrect package I was signed up for and I was never assisted. I was being debited a bigger amount without the package upgrade. I complained and my complaint fell on deaf ears. I twice reported this ****** company on this platform, however, on both occasions I didn't get assistance and so I decided to cancel the plan. As we know, it was a month to month contract, so I decided not to pay for the following months so that I don't get their service anymore because I had moved to another service provider. In recent days I've been getting emails stating I owe around R4000+ ... How does one get billed that much without 1, the issue being resolved and 2, the plan was cancelled? Fix your company and customer service, I am not going to pay a single cent because of your negligence!