Over billing contract and bad service

I took a Tablet contract with MrP mobile early May for the amount of R129 monthly with 300MB that was our agreement but now they putting extra charges which I didn't agree on they claiming that its how they operate which I did not agree on I end up paying R181,51 because of the extra charge I don't know about and they claiming that they can't cut off their extra services that they have put on my contract because its how they operate, the strange thing is they never notify that the data is exosted yet they claiming that they added extra data and airtime I don't even know when it was added. MrP mobile is *********** people's money, I regret the day I took a contract with MrP mobile . I want to cancelled it because of bad services they provide and they never attend to any complaints