You can't!
Once someone has entered an email address into a Nabble forum it stays in the record.
Similarly, a Forum Administrator has no way of removing anyone from the "Registered" group.
Both supplying an email address and then confirming a user controls that address (which lifts a user into the "Registered" group) are entirely in the hands of the user and not controllable by a forum Administrator.
In exceptional circumstances I have seen Nabble staff remove a user from a forum, but normally there is no reason to do that.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.