Prompted by this question, it appears there are two methods of disabling user accounts permanently: deleting and "destroying"
What are the differences between the two and what are the criteria that determine which method is used?
"Deleted" users have their display name denormalized, their account deleted, and all of their negatively rated posts will be deleted. Anything with a score of 0 or higher is retained with their "ghost" user. This is primarily for scenarios like a user requesting their account for deletion, and it has some degree of reversibility.
"Destroyed" users have all that, but have all of their posts deleted. It is intended for things like Spam users where we just want them to be gone.
A user must have less than a certain amount of reputation in order to be destroyed. Past that, it's up to the moderator to simply select "delete" or "destroy" from the menu.