Apple - Disable spellcheck and autocorrect in Notes forever

Edit>Spelling and Grammar>Correct Spelling Automatically 

This worked for me. It is sometimes unclickable, if that is the case click a note and write something in this. This action will make it clickable.

Autocorrect is controlled by the NSAutomaticSpellingCorrectionEnabled key. You can set this in the Notes app preferences for it to apply consistently.

defaults write com.apple.notes NSAutomaticSpellingCorrectionEnabled -bool false

Reset with defaults delete com.apple.notes NSAutomaticSpellingCorrectionEnabled.