[jQuery] ui sortable - cancel action if list is too long

[Robert Bobrowski]

I have few lists with ui sortable/droppable and I want to have let say
max 4 elements in one. So:

$("ul[id^='sortable']").sortable({
        connectWith: '.user-list',
        dropOnEmpty: true
})

$(".accept-drop").droppable({
        drop: function(event, ui) {
                //some code

                if($('#sortable'+new_id+" li").size()>4) {
                //DO SOMETHING TO STOP SORTABLE
        }
});

What code should be there? I've tried to change class to .disabled but
it blocks moving.
$('#sortable'+new_id).sortable("cancel"); causes error. Script allows
to move single elements and whole lists.
I want to be able move elements from all lists but disable drop if
it's too long.

--
Regards,
Robert