Place the following file as init.el in a directory (e.g. ~/.demacs.d)

---cut here---
(setq inhibit-startup-screen t)

(defun auto-complete-text-off ()
  (message "Trying to turn off ispell completion...")
  (setopt text-mode-ispell-word-completion nil))

(add-hook 'org-mode-hook #'auto-complete-text-off)
(add-hook 'org-mode-hook #'(lambda()
                             (ispell-change-dictionary "en_GB")
                             (flyspell-mode t)))
---cut here---

Open a text file and C-h C-v completion-at-point-functions. You should see
ispell-completion-at-point in the list of completion functions

Open a .org file and C-h C-v completion-at-point-functions. You see
ispell-completion-at-point in the list of completion functions, when the
expected behaviour
is that ispell-completion-at-point is *not* in the list.

/PA
-- 
Fragen sind nicht da, um beantwortet zu werden,
Fragen sind da um gestellt zu werden
Georg Kreisler

Headaches with a Juju log:
unit-basic-16: 09:17:36 WARNING juju.worker.uniter.operation we should run
a leader-deposed hook here, but we can't yet