Edit in JSFiddle

$(document).ready(function() {

  //アコーディオンを必ず一つは開くようにする
  var $toggles = $(".accordion-toggle");
  for (var i = 0; i < $toggles.length; i++) {
    var $toggle = $toggles.eq(i);
    $toggle.on("click", function(evt) {
      var $pane = $($(this).attr("href"));
      if ($pane.hasClass("in")) {
        console.log("表示されてるので閉じないようする");
        evt.preventDefault();
        evt.stopPropagation();
      }
    });
  }

});

<div id="accordion" class="accordion">

    <div class="accordion-group">
        <div class="accordion-heading">
            <a class="accordion-toggle" data-toggle="collapse" data-parent="#accordion" href="#pane1">toggle1</a>
        </div>
        <div id="pane1" class="accordion-body collapse in">
            <div class="accordion-inner">pane1</div>
        </div>
    </div>

    <div class="accordion-group">
        <div class="accordion-heading">
            <a class="accordion-toggle" data-toggle="collapse" data-parent="#accordion" href="#pane2">toggle2</a>
        </div>
        <div id="pane2" class="accordion-body collapse">
            <div class="accordion-inner">pane2</div>
        </div>
    </div>

</div>