increasing of spending limit without my concent

My cell phone contract is a closed contract so I should not be receiving any extra air time or data unless I request for it. Every month I receive an sms informing me that my spending limit will be increased to R..???. without an option to say NO thanks, the second sms says that the increased spending limit has been successfully uploaded. I Ignore these messages because it doesn't apply to me, but to my surprise when I see my bill OMG. I have contacted customer care several times to stop increasing spending limits on my contracts BUT this doesn't help.....the following month my bill is OMG again. My latest bill R1698.50 I'm now fed-up.<br>