Cell c fibre debited

Cell c fibre debited my bank account without my consent or a debit order. I am not even a cell c customer. I had a service with a company called greencom which I terminated in December and paid in full all outstanding amounts. Cell c took over that company in Feb and started to debit my account.

I never had a debit order running with greencom and have received no services from Cell c, so how can they debit me firstly without my consent and secondly without providing any service or goods. This is plain theft. I have emailed them all proof and they have given me a ref number but still no refund.