We are using Kunena forum with the following Security Settings:
Everybody can read the forum, but just registered users can write.
The following changes have to be made:
List of recent discussions is visible for all the visitors, but only registered users can open threads to read them.
If unregistered user click on the forum, he can see several threads where the first one, for example, HOW to find a job in your city.
If unregistered user will click on the thread to open it, he will get a message that he has to be registered to get access.