<html>
  <head>
    <style type="text/css">
      body {
          overflow: hidden;
      }
      #a {
          background-color: green;
          width: 100px;
          height: 1000px;
      }
    </style>
    <script>
      function allowScroll()
      {
        document.body.style.overflow = "auto";
      }
    </script>
  </head>
<body>
This page is not scrollable until allowScroll() is called.
<div id="a"></div>
</body>
</html>