Hmm... stumbled on a very, very interesting workaround... LOL.
I think you won't see any of those newsfeeds stuff in Active Topics anymore... please give it a try.
BTW, to be honest, I'm not too sure how active topics are computed also - I think "eyeballs" don't count, so I'd suppose you're seeing all of them appearing in active topics right now because I've just created them en masse. Over time, they should fade away (the RSS feed info doesn't get stored per se - so to the board the topics aren't updated at all - and nobody has write access to bump them).
Anyway, this workaround should work. Only issue now is that nobody will see what are the new feed sources available under "New Posts" or "Active Topics" unless they click into those subforums. No biggie though.