How do I archive a user from a plugin I want to archive users from a plugin, but can't seem to find the appropriate method to do so (not even in the control panel). I can suspend, delete, but not archive.

Any ideas?
I assume you're referring to the archived column in the craft_users table in the database.

Originally we were going to use that as a way to "soft delete" different elements in Craft (Users, Entries, etc.), but never got around to implementing it for various reasons, so there is no UI for it and no backing code to support it currently.