cancelled service in Dec,still getting charged

I canselled my Wi-Max 512 Internet service with Telkom Centurion branch on 22 Dec. I was advised that 30 days notice period app**** to which I agreed as this is their policy. In Jan they deducted a monthly sub again,I went back to the store and the consultant advised me they made a mistake,emailed accounts to advise them that the service is canselled. March another sub was deducted of my bank acc. TELKOM what do you want me to do re this matter. Is this not ***** when you deduct money off my Acc that has been cancelled 3.5 months ago.