Vodacom's refusal to cancel a contract

Under no circumstances should you ever think about taking a contract with Vodacom. Even though my contract has expired (more than 3 years old), they refuse to process the cancellation order. The contract is paid via debit order, but they want me to settle the balance via EFT and then ask Vodacom for the debit order money back when it goes through (according to the online agent). This is ridiculous. They want a month's notice, but this is impossible because there is always a balance on the account. Just another way Vodacom has found to retain contract customers against their will.