Answer to http://softwarerecs.stackexchange.com/questions/11990/chrome-extension-to-disable-spellcheck-in-a-text-field. Adds a button to stop browser from spellchecking.

spellcheckDisabler.user.js

function addButtonBefore(inputItem) {
    var button = document.createElement("button");
    var text = document.createTextNode("disable/enable spellcheck");
    button.appendChild(text);
    cssString = "font-size: 9px; border: 0; border-radius: 10px; outline: none; color: inherit;"
    button.style.cssText = cssString;
    button.setAttribute("title", "disable/enable spellcheck")
    button.addEventListener('click', function(e) {
        inputItem.spellcheck = !inputItem.spellcheck;
        e.preventDefault();
        e.stopPropagation();
        e.cancelBubble = true;
        e.returnValue = false;
        return false;
    });
 
    inputItem.parentNode.insertBefore(button, inputItem.nextElementSibling);
}
 
Array.prototype.forEach.call(document.getElementsByTagName('textarea'), function(inputItem) {
    addButtonBefore(inputItem);
});