How can I prevent the browser's default history back action for the backspace button with JavaScript?

You don't need return false or e.stopPropagation(); neither will make any difference in a listener attached with addEventListener. Your code won't work in Opera, which only allows you to suppress the default browser behaviour in the keypress event, or IE <= 8, which doesn't support addEventListener. The following should work in all browsers, so long as you don't already have keydown and keypress event handlers on the document.

EDIT: It also now filters out events that originated from an <input> or <textarea> element:

function suppressBackspace(evt) {
    evt = evt || window.event;
    var target = evt.target || evt.srcElement;

    if (evt.keyCode == 8 && !/input|textarea/i.test(target.nodeName)) {
        return false;
    }
}

document.onkeydown = suppressBackspace;
document.onkeypress = suppressBackspace;