It's been a while since your post, but I wanted to offer some insight. Canceling your parents' NBN account with power of attorney should be doable, but it might require some paperwork. Have you contacted their NBN provider directly to inquire about the process? They should be able to guide you through the steps needed.
If you're still looking for info, I found this article on selecting a whistleblower attorney, which might provide some useful tips for navigating legal matters: https://www.jdsupra.com/legalnews/6-keys-to-selecting-a-whistleblower-1291783/ While it's not directly related to canceling NBN accounts, it could offer some helpful insights.
