Create a new group call "restricted users" assign every forum to that group as a "resource"
with the exception of the forum you want the user to be able to see.

Then set all the permission of that group to NO... 

and add that user to the group.

1. Go to the admin control panel.
   
2. Click on the "Groups Manager".
   
3. Create a new group
   
   
   
   1. Give a new group a name ex. "Restricted".
      
   2. Using the multi-select add all forums as resources to this group with the exception of the forum you want the user to be able to view.
      
   3. Set "Inherit From" to "Global Registered Access"
      
   4. Click "Add" bottom
      
   5. The group was now created and appears at the bottom of the groups list. Because of a bug, when you click on edit beside the group you won't be able to see the forums assigned to this group. This will be fixed shortly.
      
   
   
4. Return to the forum's main page
   
5. Click on the "Group(s) Manager" link
   
6. From the select at the top choose the group you've created and click on "Edit Group" button.
   
7. In the "Member" textbox enter the login of the user and click "Add Member".
   
8. Now you need to restore the Anon & Regged access to the forums, you do this very simply.
   
   
   
   1. Edit the "Anonymous" user and give him "View" access.
      
   2. Edit "All Registered Users" and give him View/Post/Reply/Create polls/Attach files/Vote/Rate threads/Use smilies/Use image tags access.
      
   3. This step will no be required in future versions.
      
   
   
9. That is it, you are now done.