When a non-registered user makes a post to a Nabble forum then a cookie gets set on that device so the same username will appear next time the same forum is visited.
Obviously, individual registration is much to be preferred as the cookies then become set for each user.
If this is impossible, then I believe the only solution would be to clear the Nabble cookies on the device before each visit to the forum.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.