Disable spell-checking on HTML textfields

Yes, use spellcheck="false", as defined by HTML5, for example:

<textarea spellcheck="false">
    ...
</textarea>

Update: As suggested by a commenter (additional credit to How can I disable the spell checker on text inputs on the iPhone), use this to handle all desktop and mobile browsers.

<tag autocomplete="off" autocorrect="off" autocapitalize="off" spellcheck="false"/>

Original answer: Javascript cannot override user settings, so unless you use another mechanism other than textfields, this is not (or shouldn't be) possible.