Suggestion:
On the Leaderboard - highlight or asterisk inactive players.
You should be considered inactive if you have not logged in or played in a certain amount of time - I suggest you are inactive if it has been 60 days.
In addition, you should drop off of the leaderboard all together once you are inactive for longer - I suggest 6 months for this.
Without making a player inactive, or drop off - someone could stay there forever, but not really be in the game.