Is this limited 1 topic per user?
If it isn't, may you add such option?
May you add the possibility for the user to always be able to view that topic even if he is perma-banned?
May you add the possibility of that topic working as a regular one where users and authorized people can discuss together the situation?
Additionally, I'd like to point you to pay attention to this extension:
viewtopic.php?f=456&t=2283001
It may have some information you can use on your own project.