I've just spent the last ten minutes writing code to fix this, only to find that I didn't actually have the problem in the first place. Please try doing the following for me:
- Set all forums to display in the node list.
- Refresh a front-end page, and open the quick nav menu. All nodes should be shown.
- Set one forum (preferably one with some children) to be hidden in the list and save it.
- Refresh a front-end page and open the quick nav menu. The node / forum you specified and all its children should be hidden.
Can you confirm that this is what you see?