Car I feel for you and your family. Its a tough situation.
I have a sister in her late thirties who is also intellectually disabled, and for the last number of years has been living on her own following the death of our parents.
The situation you are facing is exactly the kind of thing I worry about. Thankfully my sister would still run things by me before committing to something but she has also been known to run the idea of buying something by someone else who she figures will agree with her because she knows I'll probably dissuade her.
I had inquired into guardianship and discovered that this only applies to minors (under 18). The are only two other options left 1. is enduring power of attorney or 2. having her made a ward of court.
Option 1 has problems in that your relative (my sister) would have to set it up, which means she has to show she is a capable person. You would then have to show at a later date that her mental state has deteriorated and she is no longer capable. Obviously this is problematic if the family feel she currently isn't capable of managing financial tasks
Options 2 is a very drastic option and would mean that all decisions would have to be made by the courts, not really a workable solution.
I find that I fall back on the hope that my sister on the whole interacts with moral, sensible people who would know very quickly that she isn't the usual customer and wouldn't try and exploit her. You will find a lot of advocacy groups online who would suggest that people like my sister have the same right to "waste" money on a phone as any other person, there are plenty of "normal" people who sign contracts with phone companies that they shouldn't.
But at the end of the day I know its me my sister will come to, to fix any problems that arise. So on the whole I try to keep the lines of communication good, try not to say no outright and get her to justify what she wants to do. I also try and stay knowledgeable about the people in her life who she might be turning to for advice.
I think the phone company have taken a bit of a hard line but maybe its a chance to teach her a lesson if you can figure out a way of doing it that will get the message across in a positive way.
Is she involved with any support groups ie. Enable Ireland, John of Gods, Step, Cheeverstown House, St Michaels House? They might be able to give you some guidance on how to approach all this.
As I said at the start I know where you coming from and unfortunately the worry never goes away