If you track a member, everytime they post it will show up in your "my tracked activity" . To undo this, remove them from your tracked list.
http://www.chess.com/home/alerts.html
The names of people you are tracking are at the bottom of the page, just click where it says "remove".
On My home there is the section called tracked activity. How do I make something become untracked? It's got to be there I just can't find it.