setInterval 启用和停止，见代码

<title></title>
    <script src="Scripts/jquery-1.4.1-vsdoc.js" type="text/javascript"></script>
    <script src="Scripts/jquery-1.4.1.js" type="text/javascript"></script>
    <script src="Scripts/jquery-1.4.1.min.js" type="text/javascript"></script>
    <script type="text/javascript">
        $(function () {

var iCount = setInterval(GetBack, 3000);

function GetBack() {

alert("aa");

$.ajax({
                    type: "POST",
                    url: "WebForm4.aspx/GetString",
                    dataType: "text",
                    contentType: "application/json; charset=utf-8",
                    beforeSend: function (XMLHttpRequest) {

},
                    success: function (msg) {
                        alert("ff");
                    },
                    error: function (msg) {
                        alert(msg);
                    }
                });
            }

$("#cOk").click(function (e) {
                clearInterval(iCount);
            });

});

</script>

<div>
        <a href="#" id="cOk" >sss</a>
    </div>

后台代码

------------------

[WebMethod]
        public static string GetString()
        {
            return "aa";
        }