moodle/lang/it/help/forum/allowdiscussions.html

<P ALIGN=CENTER><B>Allowing new discussions</B></P>

<P>For most forums you will want to allow non-teachers to start new 
   discussion topics (threads).

<P>Sometimes, however, you will want to disable this ability.  For
   example, this is useful for the News forum when you only want 
   teachers to post new items that appear on the course main page.
   Students will still be able to REPLY to existing posts though.