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>");
}