I have just attempted a test to see if I could find a way to delete a message posted as an unregistered user and think I may have found a bug in Nabble's system.
I'm not sure about this but as Nabble plants cookies on your computer when you post then if you are using the same computer as you did when posting the message then there is a chance that you could register that username used on the original post and then gain access to the Profile screen where you could delete or edit your earlier message.
If that is not possible, or it does not work, then your best bet is to contact the Forum owner (Check the bottom of the forum home page, and the link where is says "Created by..." You'll be taken to a screen that includes a link to an email screen) and ask them to delete the message, or at least edit the message contents.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.