overcharged on my account

I am extremely unhappy with my December account received from Vodacom. I requested that my line be blocked when it reaches a certain limit. This was working fine and my monthly bill never exceeded R400 until I received my December account that is. R1297.32 is what reflected on my bill. On querying this with the call centre, I was advised that they had a system problem and my line remained opened allowing me to exceed my data usage. The matter was investigated and my account was credited with a measly R462 leaving me to pay a bill over R800 which is double that of my normal monthly bill. My debit order has since gone off, leaving me out of pocket. Cassandra's explanation does not satisfy me. I want to scream out LOUD!!! I cannot wait to cancel my contract once it expires. A clear David and Goliath situation!!!