You can make your forum private and then follow the instructions below to make your forum hidden from search engines as well.

1. Log in to your Website Toolbox account.
2. Click the Integrate link in the main menu.
3. Click the Custom Code link.
4. Copy and paste the following code into the Forum Head Tag HTML Code section:

   <meta name="robots" content="noindex, nofollow">
   

5. Save your changes.