There are two ways you can expire an active Glympse. From the Recents screen, tap an active Glympse from the list to reveal options for expiring, quickly extending it or otherwise modifying it. To expire the Glympse, simple tap the "Expire" button (see below).
The other way to expire an active Glympse is to Modify the Glympse and "unwind" the timer (i.e. set the timer to zero minutes) and tap Update. This will immediately update the Glympse so that there is no time remaining on it, effectively expiring it so that your location no longer updates.