Cell c *****ing data

I honestly have had it with my contract and i don't even know what to do and i refuse to be bul**** into changing my contract or switching networks. its the 5th time in a row now that I've used more than 2Gs worth of data in a couple of days. Ive switched all my apps to only use wifi as well as make sure all apps are closed yet it still uses data even when I am on a wifi network. The first two times cell c admitted it was a fault on their side and returned my money now they refuse to acknowledge its their fault and I am just sick to death over fighting to get it back. All this while they are trying to force me to upgrade and change my contract just seems to convenient. I am over cell c now and i just want them to get their act together and sort my contract out!