Please extend a new 'deleteItem' method to achieve this functionality.
$.extend($.fn.combobox.methods, {
appendItem: function(jq, item){
return jq.each(function(){
var state = $.data(this, 'combobox');
var opts = state.options;
var items = $(this).combobox('getData');
items.push(item);
$(this).combobox('panel').append(
'<div id="' + state.itemIdPrefix+'_'+(items.length-1) + '" class="combobox-item">' +
(opts.formatter ? opts.formatter.call(this, item) : item[opts.textField]) +
'</div>'
)
})
},
deleteItem: function(jq, index){
return jq.each(function(){
var state = $.data(this, 'combobox');
$(this).combobox('getData').splice(index,1);
var panel = $(this).combobox('panel');
panel.children('.combobox-item:gt('+index+')').each(function(){
var id = $(this).attr('id');
var i = id.substr(state.itemIdPrefix.length+1);
$(this).attr('id', state.itemIdPrefix+'_'+(parseInt(i)-1));
});
panel.children('.combobox-item:eq('+index+')').remove();
})
}
})