Agreed a monthly price for 2 years so stick to it!

I have quite a few contracts with Vodacom. This past weekend I started receiving emails and SMS messages from them saying that all those contract prices will now be increased. I agreed to a price for 2 years. Can they just go and change these prices now mid term? I'm sure something in the customer protection act must prevent this. This is probably why they force us into a 2 year contract in order to have control over us. I have always been more than happy with the service I got from them but try to trick me into something and I quickly get discouraged so long and short I don't see myself renewing or taking out any more contracts with them ever again if they cant even honor the price I agreed with them for the 2 year period. Phoned their call center yesterday and their agents just say to you that's what they decided to do. Nice way to treat your more than a decade long customers!