Ah! You confused me by saying you had an email from your users. I think of that one coming from Nabble (although, obviously, it is generated as a result of user action) and, of course, you have a forum, or sub-forum, that has View permission removed.
As far as I know, that mail is only generated when someone actively attempts to enter a "hidden" forum. Depending on the structure of your forum and any sub-forums, It is perfectly possible for people to register, and so appear on your "People" list without attempting to access the "hidden" forum. Could that the the situation in your case? If so I have a post at:
http://support.nabble.com/Moderating-free-forum-tp6873076p6873871.htmlwhich explains the difference between groups such as "Anyone", "Registered", "Member" that may be of help.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.