[andygrif]

I think that the barring is put in place if they suspect that the phone might be being used fraudulently or they think you're not good for the credit.

If you'd had your phone nicked and the theif ran up and £200 bill before you notified them, you'd be grateful that they barred the line before it cost you more than that.

Saying that, I think it is appropriate to use some common sense, phone the customer, run through security checks to verify your identity and work out the reason for the abnormally high bill before just cutting you off.