I don't run any mail list archives, so can't speak with authority, but I suspect you can't remove that option.
I believe that the assumption at Nabble is that either:
# You will want to keep the archive private and you will only allow existing subscribers to view it
or
# You will allow non-subscribers to view to encourage archive visitors to subscribe (hence the options presented).
Bare in mind that no one will be able to post via Nabble unless they ALSO subscribe to the list.
So what is your reason for making the archives public - if it's not to help attract subscribers?
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.