var timeout;
var pressed=0;

    function hideNotUsers() {
        var filter = $('#users').val();
        $("#list:first option:not(:contains("+filter+"))").addClass("hidden");
    }
    $("#users").keyup(function () {
            if(!pressed) {
                pressed=1;
            }
            else {
                clearTimeout(timeout);
            }
            timeout=setTimeout(hideNotUsers,300);
    });