Even if the Nabble team were prepared were to help you with this, it seems pretty pointless and that alone may be a reason for not doing it. Their reasons might include:
Most of the message is quoted in a reply and the poster of the reply may not want his post deleted.
The GIMP User mail list is mirrored to several other forums so the post will continue to be displayed in those locations.
Potentially, thousands of subscribers to the list may have a copy of your post saved on their computer and could repost it at any time.
By referencing an old post you have, in effect, extended it's life by another two years.
My own view is that this is one of those things best ignored. Put it down to experience and should anyone ever raise it (Unlikely!) explain that you were only 12 at the time. We all do things we later regret. It's one of life's lessons that we all have to learn and learn to live with.
Just a Volunteer Nabble Helper - because the nice folk at Nabble have helped me!