Delete or archive the old iWork, which was merely moved during the update process, and is still being seen by software update. If you're still using the old iWork, then that's going to remain until you stop using it.
On second thought, the better option is to remove the old iWork from being seen by Spotlight, which should also make it invisible to the MAS. Just drag the folder or apps from the old version to the privacy tab under Spotlight in System Preferences.