Not cancelling debit order

Mondo promised me a 1 gig a month contract with a nice tablet but the tablet never could access the MTN network and I never could use any of the data. Finally they agreed to cancel my contract and pay back the money which they did, but a month later I look at my bank statement and again the debit order goes off, after they promised me the debit order was cancelled.. I don't know what to do anymore.. :(((