Charging me for a service that I cancelled and have confirmation of

Since March 2021 Vodacom is charging me for a service that I cancelled and for which I got confirmation of in writing. I tried resolving the matter in a Vodacom shop as well as with a very competent-sounding call centre manager over the phone, but absolutely nothing has happened to resolve the matter. I still get billed month after month. This is technically ***** with the manager that promised to resolve the matter now complicit in this ******* activity. Is this something I should go to the Ombudsman about or is the police / a lawyer a better option?