This is a function of Joomla's menu module, as opposed to SMF or the bridge.  Open your site modules and find the entry for your main menu.  Open the menu entry to edit it and look on the right hand side.  You will see a list of all your menu entris.  At the top is an "all" selection, which is probably highlighted, so this menu module is displayed for all menu entry choices.  You need to select each individual menu entry of your main menu choices except your fourm.  That way the menu is not displayed when the forum is selected.  Repeat this process for each of your menus where you do want the main menu to be displayed ("home" on the top menu, for example).  You many need to repeat the process for other modules if you have multiple menus (or other modules) displayed.

I tried doing what Dale L. Brackin has suggested, but it was a pain having to do the same again when I added a new menu item.

So, I dug into the template system. Duplicate your current template, and assign the new one to the forums. Then, add the PHP code in order to remove all the calls to the module that is serving the menu, usually "left".

Hope it helps