title: jQuery UI Sortable - prevent dropping on one particular list
body: <p>I have 3 unordered lists (say, <code>#list1</code>, <code>#list2</code> and <code>#list3</code>) and currently, users can freely move any <code>&lt;li&gt;</code> between those three. How would I prevent any item from being dropped in <code>#list3</code>?</p>
  
  <p>Users should be able to move items freely between <code>#list1</code> and <code>#list2</code>, as well as being able to move items <em>from</em> <code>#list3</code> into any other list, but not to drop an item into <code>#list3</code> (regardless of which list it originates from).</p>