I clear out entire sections from my “whats new” by going to the topic and clicking on “mark all read” for the topic. Be careful to not click on “mark forums read”; that will clean everything from your “whats new”.
Another way to remove one item from the “whats new” is to click on the area showing the time of the last post to that topic, e.g. “3 minutes ago” or “yesterday at 2:21 AM”. This will bring you to the last post (a pain, I know); then the thread will no longer be on the “what new”.