jQuery tag-it only allow available tags

i use jQuery tag-it in my script:

$("#user_autocomplete").tagit({
    tagSource: function (request, response) {
        $.ajax({
            data: {
                term: request.term
            },
            type: "POST",
            url: "/site2/message/users.php",
            dataType: "json",
            success: function (data) {
                response($.map(data, function (item) {
                    return {
                        label: item,
                        value: item
                    }
                }));
            }
        });
    },
    tagLimit: 3,
    autocomplete: {
        delay: 0,
        minLength: 1
    },
});

In this case, all input fields are confirmed. But I want only the fields that are available to be added. How to do it?