Keep on raising the spend limit on phone

For the third time since the contract was taken out last year the spend limit on my account( the amount of money avaliable on credit when the data minutes etc have run out). The previous two times I got warning messages that the limit would be raised and had the chance to decline. My brother however missed the sms so his spend limit was automatically raised. Wespent a lot of time the last two times this occured to undo and this time there was not even a warning sms! They just sent me a sms saying it was being adjusted with an extra R200. I have never asked for this service at any point and I am frustrated that this is happening once again