I have changed the status of one of my forums from public-no moderation, to public-moderated. The icon changed and the forum is now listed as public-moderated. All good.

Now I want to change the status of an existing user. I go to "User Settings" and maximize "Admin Settings" where I see a checked box describing this user as a "Trusted user." But the check box is grayed out; I cannot uncheck it. I've tried this logged in as admin and as host.

What am I doing wrong?

I'm logged in as admin. On the user settings, under admin settings, I see check boxes for "trusted user" and "lock trust". Currently, the "lock trust" check box is unchecked. The "trusted user" check box is checked and is also grayed out so that I cannot change it. Checking the "lock trust" checkbox and then coming back later does not allow me to uncheck "trusted user." I have tried this scenario logged in both as admin and as host. No luck. What else can I try?

Try a look in the General Settings of the Forum Admin for trust settings.   Perhaps the user manual 4.4.4 can help you to find out this.  You can download it from the Forge\Project:Forum Download page.