To hide a past event from appearing to attendees in the app, go to the CMS and click on your event from the top right-hand part of the screen.
Next, click on 'Manage Users & Events'.
Note: you must be an Admin to view the Manage Users & Events button and hide events.
Find your event you'd like to hide by locating it under the list events or search for the event by typing the name in the search bar. Then, click on the mobile phone icon to the right of "Show Managers".
The icon will change to show that it's crossed, indicating that it has now been hidden from the app.
Note: keep in mind that attendees only have visibility into the events they've been added to (i.e. they are listed on the attendee list).