Even a forum administrator cannot remove a user from the system. There are a number of options.
1. Treat the forum you have as a test forum. Either delete it or abandon it, when it will eventually be deleted by Nabble through lack of use, then create a second forum for live use.
2. Tell your users you have already created profiles for them. Tell them the password you used and warn them they are about to get an email from Nabble saying their email address has been changed. Tell them to click the link in the email to confirm the change and how they change the password to one of their choosing.
A long time ago I did see Nabble respond to a request to remove users from a forum. If you rely on that you would need to post the URL of the forum.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.