I think here is still a permission issue that can't be set properly in the dashboard. On my board as well as here it's a security concern if regular members can stop indexing of discussion by search engines.
it also allows members on the http://vanillaforums.org/ to noindex as well unless i have special permissions. when i click the gear here on a discussion: http://vanillaforums.org/discussions I can noindex any of the discussions.
The plugin checks for one of Garden.Moderation.Manage and Garden.Curation.Manage permissions. So as long as your members do not have one of that, they should not be able to add NoIndex meta to a discussion. I would assume that here on this forum only MVPs have Garden.Curation.Manage.