Impossible to cancel automatic data refill service!

I am sick to death of Vodacom. I have called 3 the call centre 3 times to try and cancel the automatic data refill service. They tell me to dial *135# and select services, then subscriptions or data refill, but none of these options exist???? And i was told they cannot do it on their side?? This is ridiculous!! Nobody seems to know what is going on!!! I've been a customer for over 20 years, but I will be cancelling my contract!