You need to contact the administrator of the Nabble archive, quoting the URL of the topc, and ask them to delete it. Click on the "Created by" link at the bottom of the to reach a screen to email them.
If the administrator does not respond then post the URL here and one of the Nabble team may be able to help.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.