A sinking ship

Telkom is like a cult, once you're in, you can't escape. I've been trying to cancel my Telkom subscription without having to go into a shop. How can they phone you and sell you a product but you have to physically go into a shop to cancel the agreement? Also, they keep spamming me, calling up to 4 times a day trying to upsell me. Yet none of these people will help me to cancel. They also refuse to remove me from their contact list. I realise the ship is sinking and this type of service non-delivery is just one of the reasons for that.