Up until Mountain Lion, you could hover over the Install or Download button in the App Store Purchases list and an x would appear. Clicking this x would hide the app, removing it from the purchases list.
Now, in Mountain Lion, you need to right-click or Control-click and choose Hide Purchase from the one-item contextual menu that displays.
To unhide an item, click on the Account link on the main page of the App Store, then go to iTunes in the Cloud > View Hidden Purchases.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120828071619773