Find your "Purchased" history in iTunes... There's a direct link to it under "Quick Links" on the main iTunes store page within the iTunes app... then select the category of media type... hover over an item you wish to do something about and hit the "x" in the circle in the upper left corner... that will 'Hide' the purchase...
If you ever want to 'Unhide' a purchase, you sign-in to your iTunes account, and in the 'in the cloud' section, manage hidden purchases.