Hi, 

my forum is just for paying members and I want to hide all its contents from non-members (who might get the link from a member) as well as from all search engines. How can I do so? Will I need a plugin or is it possible to do so from the settings? 

Okay, I found the robots.txt settings - and uploaded a robots-file. However, it does not work the way I want it (nothing should be visible except for the front page of the forum with all the sub-forums). Any help would be highly appreciated. 

You would simply edit the guest group in the admin CP and set "can use site" to off. 

So easy! It worked like a charm! Thank you ever so much, @Marc Stridgen! 

You're very welcome! Glad we could be of assistance here.