No communication on contract renewals

I am a Vodacom contract customer and have been for the past 9 years. I noticed that in January my monthly payment was increased from ~R261 to R420 (no device contract, just airtime) without any notification from Vodacom. When I called the call centre to query this, I was told that this was because my previous rate was on a 24 month contract at a discounted price which expired in December and that I should have known that and therefore they do not need to let me know of the increase. When my contract originally expired 2 years ago, Vodacom pestered me on a weekly basis to renew my contract to one at a higher price. I find it quite cheeky that because Vodacom automatically benefits from the \out of contract\" price increase this time round, it didn't find it necessary to get in touch with me at all since it was receiving more money from me. I am now very keen to cancel this contract with Vodacom and would appreciate if you could let me know how to do this. I queried through Twitter on 13-Feb and still haven't had any response...again, guessing because it's cancellation related there's no real urge to respond to a customer query. Just disgusting."