Vodacom contract a nightmare

The issue that I have experienced was: i entered into a contract with vodacom on 12 August 2016 which was for 24 months,it was for the handset and the R150 airtime i receive and i was paying 469 before the vat increase and now im paying 472,i cancelled the contract on 5 june 2018 as per their communication that 21 days notice ned to be provided,the fones were delivered on 12 august 2016 and a fist pro rata deduction was made on 16 August 2016, and on 15 september 938 was deducted again,according to my understand the 24 months period end on august meaning the last deduction should be on august ,now vodacom want to bill me for september 2018 which make it 25 months now and that is not acceptable because i have paid for the pro rata when i got the phones and the handset has been paid off august 2018 was my last deduction

It occurred on: August 6th, 2018

I want:vodacom to cancel the deduction that will be made on 3 september 2018