Alright. We somehow seem to have completely overlooked something basic in Prison 3:

/ranks demote [player]

This command will move the player down the ladder, and should be a moderator only command.

Closing this since rank commands have been added to the upcoming v3.2.1 release.

/ranks promote [player] [ladder]
/ranks demote [player] [ladder]
/ranks set rank [player] [rank] [ladder]