How can one turn off spell checking in Notepad++?

You can do this in one of four ways.

1. When installing; during the Choose components selection navigate to "Plugins" and uncheck the "Spell-checker" plugin.
2. In Notepad++ locate "Plugins" > "DSpellCheck" > "Auto-check document"
3. Alternatively for no 2, press the key combo Alt+A.
4. Press the toolbar button labeled DS or ABC (from version 6.5)

You can click the "DS" or the button (or Alt+A) to turn Spell-Check off/on easily:

The instructions are right on the download page:

If it does bother you, just uncheck Menu item Auto-check Document (Menu: Plugins -> DSpellCheck -> Auto-check Document).

Source

There are TWO Settings that are independent of each other: Auto-Correct/Completion and Spell-Check. "Auto-Correct" can inadvertently change what you intended to write, while "Spell-Check" just independently and automatically marks misspellings.

To change AUTO-CORRECT/COMPLETION:
-- Go to Settings... Preferences...
-- Then, on the left pane, (under the "Backup" section), go to "AUTO-COMPLETION" section:
-- Simply UN-check this choice. (No NPP++ restart required)
This does not turn off spell checking, so you will still see misspelled words marked with a red underline.

To change SPELL-CHECK:
Simply toggle the , (or the DS button in older versions.) NOTE: This will NOT change the "Auto-Correct/Completion" setting. (This icon will not show up for me when "big icons" are chosen, only "standard icons". Also, couldn't find the Alt+A as a shortcut, either. YMMV, but at least it is still under the Plugins/DSSpell menu as an option, assuming you've enabled menus as visible.)

No, you are not alone.... This "Auto-Correct/Completion" problem really "bugged" me until today.

In the January 2015 version 6.7.4, by default, "Spell-Checking" now does a lot more than just "check" spelling. Auto-Correct/Completion has been driving me mad, occasionally fixing my "spelling" for things like precise links that now no longer work, which I didn't notice were changed, until they failed.

NP++ Devs: Thanks for all your work, but I'd suggest putting an on/off on the toolbar for both Auto-Correct/Completion and Spell-Check, and I'd suggest that you turn Auto-Correct/Completion off by default. Also, in preferences, call it "spell-checking" so we can find it, and allow me to change both settings under one place. If that means that there are two places in the UI or prefs, then allow me to change both settings, both places, where it is obvious there are two separate settings. Please clarify that Auto-Correct is a synonym for Auto-Completion, (which is the most annoying problem), in addition to Spell-Check, (which merely marks misspelling).