You're in customer service. Cool. There is a difference between cutting someone off and proactively notifying them that their behaviour is getting expensive.
So, have you ever worked in IT? Ever implemented a file server without both hard and soft usage quotas? How about a mail server?
Let's work through the example.
Your Father just died in the UK. You grab your phone, take a cab to the airport and pay 3k for a flight to the UK to get on the first flight out. While in the UK, you need to make a bunch of calls. You don't have time to go down to the shop and pick up a local prepaid sim, so you use your phone and rack up a bill at $5/minute. After 12 minutes on one call, you've hit a soft limit of 2x your usual usage (usually you don't go over plan, and your plan is $30/month).
The carrier automatically puts a flag on your account.
You finish your call and hang up.
The carrier's IN detects the disconnect and creates a call between you and the call center.
The call center staff verify your identity and have a discussion about overseas charges. They sell you an overseas roaming plan and (optionally) retroactively apply it. At the very least, they tell you that you've just spent $150, and see if you're surprised. Something like this:
"Hello, this is Jim from TNZ, we've noticed some dramatic changes in your account. You've just spent $150, so we would like to verify your identity. Can you please tell me your name, DOB and and home address?"
"Sure thing, Bill, Oct 1, 1980, 3 Wibble St, Auckland"
"Thank you Mr Jones, I see you're making a lot of calls in the UK, currently you're spending $5/minute, but if you switch to plan X, you can avoid most of those charges. Would you like to switch?"
"Wow, I didn't know that, yes please switch me."
"There, that's all done, is there anything else I can do to help? Will you be using the mobile for data while you are there? Those charges tend to surprise as well."
"No, just calls and SMS. Thank you"
"Very good Mr Jones, please call us if you have any problems"
Let's say that the telco can't get in touch with you. Eventually you hit the hard limit (let's say 5-10x monthly spend). The system then declines your calls, redirecting any outgoing attempts to the call center, as well as spamming you with SMS for every incoming call.
Hard to do? No. In fact, any carrier running eServGlobal's ChargingMax (shameless plug) product are able to implement that service out of the box (except perhaps the retro-active plan change - maybe even that!). The hardest part is coming up with the script for the call center people.
At the end of the day what your asking for is Telco's to take responsiblity for someone elses phone usage. It isn't hard for the customer to call to report their phone is stolen.