All the instructions are here http://jqueryui.com/sortable/#items
In your code that I extracted and put here http://jsfiddle.net/axgTF/1/
change
items: "li",
to
items: "li:not(.dontsortme)",
and add that class to the items you want to disable from sorting, e.g.
<li id="list_10" class="dontsortme"><div><span class="disclose"><span></span></span>Item 5</div>
Which you can see working here http://jsfiddle.net/axgTF/2/ (Item 5 is disabled)