Log Out In Java Script

This happens because of cache.
1. clear the sessions
2. clear the cache such that browser has no history (this will make back/forward button in browser grayed out disabled.)
code for clearing cache can be put up in code behind as follows:

// Code disables caching by browser. Hence the back browser button
// grayed out and could not causes the Page_Load event to fire 
Response.Cache.SetCacheability(HttpCacheability.NoCache);
Response.Cache.SetExpires(DateTime.UtcNow.AddHours(-1));
Response.Cache.SetNoStore();

OR

You can add somethin similar in form aspx if you want to place it there:

<meta http-equiv="Cache-Control" content="no-cache">
<meta http-equiv="Pragma" content="no-cache">
<meta http-equiv="Expires" content="0"></meta></meta></meta>

OR

You can clear browser history through JavaScript....

//clears browser history and redirects url
<SCRIPT LANGUAGE=javascript> {  var Backlen=history.length;   history.go(-Backlen);   window.location.href=page url }</SCRIPT>

OR

Page.ClientScript.RegisterStartupScript(this.GetType(),"cle","windows.history.clear",true);

OR
as you say in you logout event:

protected void LogOut()   
{       
     Session.Abandon();       
     string nextpage = "Logoutt.aspx";       
     Response.Write("<script language=javascript>");             
     Response.Write("{");       
     Response.Write(" var Backlen=history.length;");       
     Response.Write(" history.go(-Backlen);");       
     Response.Write(" window.location.href='" + nextpage + "'; ");
     Response.Write("}");       
     Response.Write("</script>");   
}