Seems like there could be other factors in them stopping it.
Vodaphone are just wanting to make money so there is little incentive in them putting small limits on their contract phones.
In this case, I'm guessing it may have raised suspicion as to the significant increase in use in such a short period suggesting the phone may have been lost/stolen and being misused.
In this instance it may be harder for them to reclaim the bill run up - not sure on their t&cs and guessing there is liability on the contract holder to pay on the basis that the phone is their responsibility and they should alert Vodaphone to any problem so they can cut it off, protecting both parties. There may be liability but they are probably more likely to encounter difficulty in getting people to pay up and have a fight to get the money back.
I'm wondering if there is an automatic flag up as they can't monitor every phone usage with the amount of customers they have, I guess like banks where certain unusual activity flags up and they stop your card until they can speak to you.
Guess it depends on how their system works and how the bill was run up and over what period. Was it gradual or a lot of activity within a very short period of time? Do you know what it was exactly that ran the bill up so much (general calls, foreign calls, premium rate services, application charges, data charges eg downloading films)?