any user can edit any post

[Lynne]

Look further down on that usergroup page under Administrator Permissions. Are either of those set to Yes? Look at the secondary usergroups also and check all these permissions.

Look in Forums & Moderators > Show All Moderators . Are any of these users listed there?

Try disabling your modifications/plugins and see if you still have this problem.
Note: To temporarily disable the plugin system, edit config.php and add this line right under <?php

PHP Code:

define('DISABLE_HOOKS', true);