NodeBB permissions are additive only, there are no negative permissions such as denying a specific user or group.
Sorry to bump an old topic; are there any plans to implement such a feature anyway? Or is there a way we can build this ourselves? Its a pain right now to organize birthday gifts and allow everyone but one person.
@UncleSam Check your DNS settings. This has nothing to do with NodeBB. Your domain doesn't seem to be pointing to the IP address. Pinging your domain is not resolving to the IP address.