Permanently delete a user. This cannot be undone. Their sign-in is removed, all organization access is revoked and their personal details are erased. Records they created, such as campaigns and conversations, are kept and still show them as the author. Their email address becomes free to invite again. Returns 409 if the user still has an enabled API key: revoke it first, or every call made with that key will stop working. To remove access reversibly, use Deactivate User instead.