function setDisabled() {
  document.getElementById("glyphicon glyphicon-share-twitter glyph-small icon").disabled = true;
}

<iframe id="ID_DO_IFRAME"></iframe>

function setDisabled() {
  var iframe = document.getElementById("ID_DO_IFRAME").contentWindow;

  var shareBtns = iframe.document.getElementsByClassName("glyphicon-share-twitter");
  for (var i = shareBtns.length - 1; i >= 0; i--) {
    shareBtns[i].disabled = true;
  }
}

function setDisabled() {
  var iframe = document.getElementById("ID_DO_IFRAME").contentWindow;

  iframe.document.querySelector("glyphicon-share-twitter").disabled = true;
  }
}

function setDisabled() {
  var shareBtns = document.getElementsByClassName("glyphicon-share-twitter");
  for (var i = shareBtns.length - 1; i >= 0; i--) {
    shareBtns[i].disabled = true;
  }
}