Easy-peasy!
Just delete the items that you don't want to appear in your purchased list.
Hiding is not too apparent, but all you need to do is mouse down the list, and click the X that appears next to the Update/Purchased list. That item will immediately disappear from your list. It's not removed forever, just hidden. If you make a mistake, or want to return a hidden item to your list, go to your account in the App Store, and you'll see a new item for viewing hidden purchases, which can return that item to your list...
Amazing, eh?