how to restrict single login with in portal

inherit ICallbackEventHandler this eventhandler it ca automatically create two methods one is return results another one is store the values and close the i mean kill the sessions.

public partial class 

//
this event handler is catch the browser information whn browser naviage or close it can handle.

public partial class sample : System.Web.UI.Page, ICallbackEventHandler
{
 protected void Page_Load(object sender, EventArgs e)
    {
      ClientScriptManager cm = Page.ClientScript;
            String cbReference = cm.GetCallbackEventReference(this, "arg", "ReceiveServerData", "");
            String callbackScript = "function CallServer(arg, context) {" + cbReference + "; }";
            cm.RegisterClientScriptBlock(this.GetType(), "CallServer", callbackScript, true);
    }
public string GetCallbackResult()
    {
        return "Success";
    }
 public void RaiseCallbackEvent(string eventArgument)
    {
       //here kill the sessions
    }

}

before writing this you can take .js file
write like this and refere all .aspx pages
function confirmExit() {
//do something
}
function ReceiveServerData(arg, context) {

}