Vodacom increases subscription fees

I've just received a message from Vodacom, saying that as of 1 May 2015 my bill will be increased by R 30.00 p/m. Surely this is ******* in the middle of a signed contract. It infuriates me that they can do such a thing. Also if I wanted to opt out of this contract, there are penalty fees applicable. I see that with the \Top Up"accounts"