The flag you see on your forum will only report topics to Nabble and it intended for those that break their terms and conditions. It is not intended for use by Administrators on their own forums.
To create a similar facility for your users to report content that break your terms and conditions, would require you to do some NAML coding of your own.
However, it would be expected that a forum's own administrator will be keeping an eye on all new posts themselves and not need such a facility. The registration and membership processes should enable an administrator to exclude those likely to post inappropriately. Forums with a world-wide interest that attract postings throughout the full 24 hour period would be expected to have a group of moderators able to move suspect posts to an area of the forum not visible to ordinary users, so a decision could be taken amongst themselves on how to handle it.
You could also create a pinned topic asking your users to post an email to you quoting the permalink of the offending post as a way of notifying you, should your members permissions not be eliminating those likely to post unwelcome messages.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.