Tariff increase

I got a message today from Vodacom that my Smart XL subscription will increase from R699 to R739 per month effective 1 May 2015. When I made an enquiry I am sloppily told that the increase is indicated in the contract that I signed, that Vodacom will increase their tariffs as and when they deem necessary. I am not happy with this attitude displayed and I have been with Vodacom since 1999 and come renewal of this contract in November I will not renew it. I know my decision might be a drop in an ocean for them but I wonder how many other drops out there to this ocean.