How forbid closinf drop-dupn menu?

<div class="btn-group">
  <button class="btn btn-mini" data-toggle="dropdown">Купить</button>
  <button class="btn btn-mini dropdown-toggle" data-toggle="dropdown">
    <span class="caret"></span>
  </button>
  <div class="dropdown-menu">
    <li><input type="checkbox" name=""/> <span>Купить</span></li>
    <li><input type="checkbox" name=""/> <span>Снять</span></li>
  </div>
</div>