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:
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....
<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>");
}