Unauthorised Debit Order: Refusal to Reverse

I cancelled my account with Afrihost for internet access which is on a month to month basis on 11 February 2020, and this is paid at the beginning of the month for the current month i.e. Feb payment will be made at the beginning of Feb. They assured me that I wouldn't be billed for March. I discovered that they had billed me for March and called FNB to reverse the amount after being sent back and forth via their Secure Online chat, the agent told me to do it myself for free, otherwise she would charge me if she did it. I tried to do it, the app wouldnt allow me to reserve the amount. I find this to be ridiculous to say the least, FNB now just can allow unauthorised payments to occur on my account, and when I request them to reserve the amount they tell me stories??? This is utterly unacceptable!