Sortable - cancel - allow just one element to move

Hi dudes,
I'm currently trying to create a sortable list which should just one
element allow to move within the list...
Here is a simplified version of my try:
jquery:
$('.menu_list').sortable({
        axis: 'y',
        cancel: 'not(#moveable_item)',
});
html:
<ul class="menu_list">
<li id="moveable_item">new entry</li>
<li>Point 1</li>
<li>Point 2</li>
<li>Point 3</li>
<li>Point 4</li>
</ul>
This doesn't work, no item is moveable/sortable. I guess i can use the
"start" option to prevent other items from beeing moved, but i hadn't
figured out how to stop a on-going sort functional. I had something
like this on my mind:
$('.menu_list').sortable({
        axis: 'y',
        start: function(e, ui) {
if (e.target.innerHTML != 'new entry') {
somecodetostopthesorting(); <?>
}
}
});
so....help? :D