How to turn off spelling and grammar check in OneNote on Windows 10

For disabling spelling and grammar errors, follow these steps:

1. Go to Review tab → Language → Language preferences in OneNote from the top ribbon.
2. OneNote options window will appear, go to Proofing, third from top, you can see the options on the right side.
3. Click Hide Spelling and grammar errors.
4. Restart OneNote.

It will disable all the spelling mistakes.

You can't. From a good article:

Note also doesn't respond to the Windows 10 Spelling feature. However, OneNote doesn't even have options to control spelling.

Another reason why I've given up on MS completely.

UPDATE: This has been changed in newer version. See this answer

I finally managed to get rid of this maddeningly annoying difficulty, i.e. not being able to disable proofing in the OneNote app for Windows 10, simply by updating the app.

The new version lets you "Hide spelling errors" under Settings → Options.

You can update OneNote via the Windows App Store.