Vodacom agents phone me consistently to tell me to upgrade

I receive a few daily calls a day from Vodacom agents telling me I am due for an upgrade. I have had as many as five missed calls from Vodacom agents on my cellphone and if I don't answer they start phoning my work phone number. I have told them a few times to remove me from the database as I don't want to upgrade, however I am just ignored. Rather try not to register with Vodacom, as it seems as soon as they have your details they wont stop bothering you.