Here's how:
****hxxp://wyy.tenforums.com/tutorials/5918-windows-defender-turn-off-windows-10-a.html****
(note - on this forum I've not enough posts to post a link!!!)
In Win 10 Defender is designed to turn back on defying the option as you've noted. Hmm.
Of course, installing a decent security solution should also disable it.
Note- it may be responsible for unwarranted highish CPU use.