In order to mark a matter as "archived" (which means it will be hidden from the list of active matters), you need to go to "FILE...Open...Browse the full list of matters". Highlight the matter in question and click the "Archive" button at the bottom of the screen.
Once the matter is marked as archived it will disappear from the list of available matters in the "Open matter" screen (unless the "show archived" button is pressed).
Here's an example:
