I'm thinking there might be a simple and fair way to thin out the number of players who've stopped playing completely, but for whatever reason have not officially exited the ladder. For instance: what if players are given 3 consecutive -1 turn boots, and then are automatically removed from the ladder until they choose to rejoin?

As I understand it, players cannot be removed entirely from the ladder unless all their past games have completely expired, because other players' scores are calculated based on the outcome of those games?
But I think auto-hibernation after any auto-boot should be implemented, to stop the just auto-booted player (who apparently cannot or will not play at the moment) from being pitted against yet another ladder player.

you can remove yourself from the ladder at any time, it just puts you into the army of not-ranked and stops giving you games.. but I believe Diab is right, you stay not-ranked for 3 months until your scores are irrelevant