Hi Andy,
This is set through the "Forum Permission" section at the bottom of the forum Modify page. From your admin panel, click "Forums", then "List All", then click "Modify" beside the forum you wish to change. At the bottom, you'll have a list of groups such as "Guest Users", "Non-Validated Users", "Registered Users", and "Moderators of any Forum", and below these, any custom groups that you have created.
So, if you want registered users to be able to create a thread, select "Read/Reply/Post access." Otherwise, if you want registered users to only be able to reply, but not start new threads, click "Read/Reply" access. "Read access" gives them read-only access to the forum.
Jason Rhinelander
Gossamer Threads jason@gossamer-threads.com