I found a hack to be able to browse server / upload documents in FCKEditor in combimation with the DNN forum (but it would also apply to Blog and other modules using the FCKEditor as well).

1. Login as Admin
2. Create a DNN Forum, and start a new Thread.
3. Use "Show custom editor options" of FCKEditor.
4. Configure the instance so that a role (such as UnAuthenticated Users) only have access to the "No Gallery" toolbar (so the image/flash chooser doesn't show for this role or roles).
5. Save configuration for the instance.
6. Logout.
7. Goto Forum.
8. Post a new Thread
9. Notice there is NO image / flash chooser, but there IS the emoticon option.  Insert a simple emoticon.
10. Right click on the newly inserted emoticon.
11. Choose Image Properties.
12. Browse the server, upload files, do as you wish.... 
13. Congratulations.

Of course, this only works IF the role ALSO has been permitted upload capabilities of the root folder where FCKEditor instance is pointed... but still something to consider.  You can see the problems here if you dig a little bit.  Best case scenario is to ONLY allow the "Basic" toolbar for any role that you don't really trust.

The only way that I can tell to ensure that the users can't browse the server at all is to do this:

1. Create a sub-folder using Admin > FileManager in your portal root.
2. Change the permissions so that ONLY Administrators have VIEW access to this folder (don't allow any other roles to have VIEW).
3. Goto the FCKEditor instance(s) on your site.
4. use the "Custom Editor options", and point the default folder to that newly created sub-folder (step 2)
5. Then, if/when someone tries any of the hacks above, it will open the image properties, but if they hit "Browse Server", it will pull up an empty folder without any upload/browsing capabilities.