VODACOME MAFIA!!! HOW MANY TIMES MUST I REQUEST A LOCK ON MY CELLPHONE ACCOUNT???

My monthly cellphone account (SM005169-3) should be R1241,59 per month. From the very start of the contract, I requested a lock on my usage since I do not want to exceed this amount. However, since the contract started more than a year ago, Vodacom ignored the "soft lock" on my account many times and allowed me to exceed this amount by hundreds of Rands. I have so often called their contact centre requesting this "soft lock" again and every time they fixed it for one month, and then the account skyrockets again. This month, my invoice is R1910.59!!! Gee whizz people - YOU ARE SUCH A BUNCH OF SHARKS!!! How can I get out of this "mafia type" relationship with Vodacom???