A bit laborious to set up but seems to work. You're actually opting in to what you want, not opting out of what you don't want.
Create a custom activity stream using "More", "Activity", "My Activity Streams" then "Create New Stream".
Select your preferences on this page, give it a name and save it. The choice for "Following" doesn't seem to suit but that will be fixed next.
Go back in to this new stream and under "Following" change it to "Content posted in areas I follow" which wasn't an option when creating a new stream. Save changes.
Go into the sub-forums you want to view and follow them, selecting if you want notified etc each time. Select your new stream and you'll see only the new stuff in only the forums you've chosen to follow. Here's the heading of the one I've just done.
They're not laid out well (IMO) doing this but gets you what you want I think.