You can't really do anything about search engines. Once they have indexed a site, they have it on file and you're doomed - subject to any local legislation on a "right to be forgotten".
If you still have access to the forum, you can use Nabble's feature at:
Options > Application > Extras & Add-ons > Content > Hide contents from search engines
This inserts a robots.txt file at Nabble that requests search engines not to index the forum.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.