Telkom Trying to collect on a contract that's run its course.

I took out a 10gb mobile wifi contract about 5 years ago, so we wouldn't need to use mobile data for social media, etc. When the contract expired, I renewed it for a further 2 years, but at the same time took out a Vodacom contract of 40gb as I started my own business and the 10gb was not sufficient. Once I realised that the 40gb was also not sufficient, we had uncapped fibre installed, thus the Telkom 10gb was no longer required. As it was a 24 month contract for a mere R99 pm, we kept it in place until it finished and duly went to Telkom in Greenstone a few months afterwards to cancel it. The lady that assisted me advised that if we cancel it, it automatically cuts it off, which wasn't a problem, seeing as we were sufficiently covered. Low and behold, the end of that month the debit came of my account. I assumed it was too late for them to cancel it, thus it ran and I unpaid it via my banking app. The next month it was the same thing, which also were manually unpaid (which does cost me bank charges). A few months went by and all of the sudden, on 25/12! it was debited again, but now it was +-R300. Once again I unpaid it. Out of nowhere, Natasha from some debt collection agency phoned me on 17/2 asking why I've stopped the payment and subsequently on 27/2 I received an SMS advising they've not received a payment. Seeing as my contract period was finished, I physically went to the store more than once to cancel it and was assured it was cancelled, legally speaking you're acting ********ly and in fact, this is harassment. I want it to stop immediately and written confirmation to be sent through.