Debit order and bounce back conflict

Horrific experience. They are truly the most unprofessional collective I have ever encountered. All they care about is their money. After my cancelation, I asked for the debit order to go off on the 15th, which went off on the 2nd of December. This left me in the minus for more than a week. I enquired it and sent my bank statement which clearly states the amount of money WAS TAKEN AND NEVER RETURNED. Now they say my account is in arrears and want more money from me. I will never recommend this unprofessional place for anyone as they will leave you bankrupt as they did with me. No thank you!