Increase of contract fees mid-term

I am very upset that Vodacom has decided to increase contract fees mid-term. To me this is *********. In my mind a contract is for a fixed term, at a specified rate for a specific service. I have written 2 emails to them and have received no response other than an emailed link directing me to their T's & C's. They have very craftily 'hidden' in their small print their right to change the terms of the contract provided they give customers 1 months notice - which is never stated when you enter into the contract with them. Also, these T's & C's state ... \by continuing to use the service you agree to the change in the contract". My question is this .... HOW do I stop using - and paying for the service - if I'm in a contract. Can I cancel my monthly stop using their services and cancel my monthly payment mid-term? Vodacom ... I've been a customer for 20 years"