1<form method="post" action="/form" autocomplete="off">
2[…]
3</form>

1$( "#tags" ).autocomplete({
2      source: availableTags,
3        focus:function(e, ui){
4            //if focusing on the extra elements return false thus doing nothing
5            return ui.item.idx<=2;
6        },
7        select:function(e, ui){
8            //if selecting on the extra elements return false thus doing nothing
9            return ui.item.idx<=2;}
10    }) .data( "ui-autocomplete" )._renderItem = function( ul, item ) {
11        //small trick to get the current element index while the list is constructing, we set this as a helper for later usage inside the item.
12        item.idx=ul[0].childElementCount;
13           return $( "<li>" )
14               //if index is greater than 2 then we add a custom 'disable' class. this will help formating the disabled elements
15               .toggleClass('disable',ul[0].childElementCount>2)
16               //appending the element
17               .append( "<a>" + item.label + "</a>" ).appendTo( ul );
18    };