Cell c hands over even if paid up and then harrass you

We cancelled our contract more than a year ago all last payments were deducted and debit order cancelled by cell c -6 months later and now for a year they handed over and want us to proof that it was cancelled but if debitorders stopped and cell c never sent a cancellation letter they put rude people on our case twice a day. The most rude person ever phoned today they are not willing to listen just threatening Cell C and CSS is really not willing to just listen or even go back to the recording of the cancellation day and they want proof from us that it is cancelled- why do a company stop debitorders- because contract ended don’t come with extra cost and willing to say what for !!!! Cell C a big no no