This looks to me as if it's a Administrator security issue, not a Nabble one!
By default a Nabble forum allows "anyone" to post on a forum. You should consider forcing your users to register before they can post. Users who are "registered" must provide you with a valid email address before they can post, giving you the same level of security as the typical mail list. If you require some higher level of security then consider requiring users to be in the "member" group. This is not an automated procedure and requires you to approve each user individually after they have met any criteria you choose to set externally of Nabble.
To institute any of these conditions, go to OPTIONS > USERS > CHANGE PERMISSIONS.
I have a post explaining a little more at:
http://nabble-support.1.n2.nabble.com/Moderating-free-forum-tp6873076p6873871.html
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.