Very old post, I know, but it still shows up on top when one searches "Delete User with API"... there are even more topics and all of them state that it is not possible to delete a user via API, but it is possible at least since 2021: https://developer.khoros.com/khoroscommunitydevdocs/reference/delete-a-user
It might have been introduced with the GDPR functionality.
You can simply issue a DELETE request (with proper authorization of course) to /api/2.0/users/<user-id> to delete a user, you'll get back a user object (JSON) with the deleted property set to true.