FNB cannot cancel debicheck mandates

i have made payment arrangement with a merchant to deduct from my account, in December they mistakenly double deducted. i called them and they told me to reverse one deduction with the bank because with them its gonna take time. i contacted FNB and they told me that the debit order reversals have been blocked, i had to wait for 2 weeks to be refunded. yesterday they did the same from my capitec account and i contacted FNB to cancel the active debicheck because i fear that they are going to double deduct and my debit orders are going to bounce, now FNB says they cant cancel the debicheck when Capitec was able to cancel it. so clearly FNB is not being useful and the person called Shanil ended our conversation while i was still asking questions, he/she is very rude and i would like to close my account and move my debit orders to Capitec