User script to prevent peeking in Duolingo. Download tampermonkey, and then click 'raw' on this page and it should install the script for you.

nopeek.user.js

// ==UserScript==
// @name         NoPeekDuolingo
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Prevents peeking in Duolingo practice sessions
// @author       Michael Snowden
// @match        https://www.duolingo.com/practice
// @grant        none
// @require http://code.jquery.com/jquery-latest.js
// ==/UserScript==

(function() {
    'use strict';
    function noPeek() {
        $(".hint-popover").remove();
        setTimeout(noPeek, 10);
    }
    noPeek();
})();