Stuart, this is controlled via the forum admin control panel's "General Settings" I believe. (I sometimes forget where things are in different versions). I do know, however, that there is a setting (checkbox) "Trust New Users" and another next to it "Lock Trust" that can be changed to do what you need.

As I remember remember right uncheck "Trust Users" will untrust all new users, not the existing setting.

And the Lock New User Trust will prevent to change the "Trust Users" feature.

So you have to set the Trust Users uncheck, and unscheck also the Lock New User Trust.  Then change all Trusted users settings to uncheck the "Trust Users".  Make sure the Forums have set to one of the moderated Forum behavior.

I'm having the same problem.

I have a forum setup with the following settings, but new registered users have their "Trusted User" setting checked, and grayed out.

1. General Settings> User Settings>

Trust New User - Unchecked
Lock New User Trust Settings - Unchecked
Unmoderated - Unchecked

2. Manage/Foums Groups> General Settings>

Public Moderated W/ Post Restrictions
 

3. Registered Users

Start Thread - Checked
Post Replys - Checked
Moderators - Unchecked

If this really is a bug, then it's a criticial bug. How about a critical update to fix it? The above post didn't work.

Thanks,

Mark