Debit a cancelled policy from my account

I have asked Nedbank in writing on the 30th January 2015 to cancel my bond ability policy on the 28th of February as i would have a new policy in place from the 1 March 2015. Then Nedbank still billed me again for March even though they confirmed that it was canceled and they refuse to give me an explanation why the bond ability went off as an debit order again at the end of February for March.