want to clear session variable value on click of signout linkbutton

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ASP.NET

Hi,
I want to clear my session variable on click of signout button.Here I am giving my code,Please check it and resolve.Thanks in advance

This is my login form code

private void Page_Load(object sender, System.EventArgs e)
        {
            // Put user code to initialize the page here
            sConnectionString=ConfigurationSettings.AppSettings["ConnectionString"];
            Label2.Visible = false;

        }

this is my submit button code of login form

private void cmdsub_Click(object sender, System.EventArgs e)
		{
			sqlTest = "select * from tbl_user where du_user='" + txtuser.Text+"' and du_pass='" + txtpass.Text +"'";
			OleDbConnection dbConn = new OleDbConnection(sConnectionString);
			dbConn.Open();
			OleDbDataAdapter sqlad=new OleDbDataAdapter(sqlTest,dbConn);
			DataSet ds=new DataSet();
			sqlad.Fill(ds,"tbl_user");
			int rowcount=ds.Tables["tbl_user"].Rows.Count;
			if(rowcount>0)
			{
                Session["loginid"] = txtuser.Text;
                Response.Redirect("ListDetail.aspx");
                Label2.Visible = false;
			}
			else
			{
                Label2.Visible = true;
                Label2.Text = "";
                Label2.Text="Your id and password do not match";
			}
			
		}

this is my suggestion form code where i have to go sfter clicking on submit button of login form.code is below
mean this is page load of suggestion form

HttpContext.Current.Response.Cache.SetCacheability(HttpCacheability.NoCache);
           HttpContext.Current.Response.Cache.SetNoServerCaching();
           HttpContext.Current.Response.Cache.SetNoStore();

if (Page.IsPostBack == false)
                {
                    sConnectionString = ConfigurationSettings.AppSettings["ConnectionString"];
                    ddlministry.Items.Clear();
                    sqlTest = "select distinct(du_suggestname) from tbl_suggestion";
                    dbConn = new OleDbConnection(sConnectionString);
                    dbConn.Open();
                    OleDbDataAdapter sqlad = new OleDbDataAdapter(sqlTest, dbConn);
                    DataSet ds = new DataSet();
                    sqlad.Fill(ds, "tbl_suggestion");
                    for (int i = 0; i <= ds.Tables["tbl_suggestion"].Rows.Count - 1; i++)
                    {
                        ddlministry.Items.Add(ds.Tables["tbl_suggestion"].Rows[i][0].ToString());
                    }
                }

This is my submit button code of suggestion.from where i choose my data from dropdownlist name ddlministry and click on submit button.
below code is for submit button of suggestion form

sConnectionString = ConfigurationSettings.AppSettings["ConnectionString"];
                sqlTest = "select * from tbl_suggestion where du_suggestname='" + ddlministry.Text + "'";
               OleDbConnection dbConn1 = new OleDbConnection(sConnectionString);
               dbConn1.Open();
               OleDbDataAdapter sqlad1 = new OleDbDataAdapter(sqlTest, dbConn1);
               DataSet ds1 = new DataSet();
               sqlad1.Fill(ds1, "tbl_suggestion");
               DataView source = new DataView(ds1.Tables["tbl_suggestion"]);
               DataGrid1.DataSource = ds1;
               DataGrid1.DataBind();

A button is linkbutton from where i logeed out and code is below

Session["loginid"] = null;
Response.Redirect("login.aspx");

Problem is that when i click on logout button then it come into login page again but if i click on back button of internate explorer then it again come in suggestion form with data.but i want that after logout i don't want go into suggestion form pressing on back button also
Please help me thanks in advance