On OS X El Capitan 10.11.6, no matter what I do, including all the suggestions I've found on Google and on this site, every time I launch the Notes app it has Check Spelling While Typing enabled.
How do I permanently disable this feature?
Open Notes and go to an existing note or create a new one. Right-click in the body of the note. A menu opens and one selection is 'Spelling and Grammar'. Clicking on this selection gives you a sub-menu with selections for spell checking. Select the one you want.