Disable ASP.Net DropDownList items using C#

Dear Sir,

I want to disable dropdown item while click on Save button.

please help me sir.

public partial class _Default : System.Web.UI.Page
{
    SqlConnection con = new SqlConnection(@"Data Source=(localdb)\Projects;Initial Catalog=ocms_admin;Integrated Security=True;Connect Timeout=30;Encrypt=False;TrustServerCertificate=False");
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            refreshdata();
        }       
    }

    public void refreshdata()
    {
        con.Open();
        SqlCommand cmd = new SqlCommand("select * from comp_box", con);
        SqlDataAdapter sda = new SqlDataAdapter(cmd);
        DataTable dt = new DataTable();
        sda.Fill(dt);
        con.Close();
        GridView1.DataSource = dt;
        GridView1.DataBind();
    }

    protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowType == DataControlRowType.DataRow)
        {
            SqlConnection con = new SqlConnection(@"Data Source=(localdb)\Projects;Initial Catalog=ocms_admin;Integrated Security=True;Connect Timeout=30;Encrypt=False;TrustServerCertificate=False");
            DropDownList DropDownList1 = (e.Row.FindControl("DropDownList1") as DropDownList);
            SqlCommand cmd = new SqlCommand("select * from complaint_status", con);
            SqlDataAdapter sda = new SqlDataAdapter(cmd);
            DataTable dt = new DataTable();
            sda.Fill(dt);
            DropDownList1.DataSource = dt;
            DropDownList1.DataTextField = "STATUS_NAME";
            DropDownList1.DataValueField = "STATUS_NAME";
            DropDownList1.DataBind();
            DropDownList1.Items.Insert(0, new ListItem("--SELECT_STATUS--", "0"));           
            DropDownList DropDownList2 = (e.Row.FindControl("DropDownList2") as DropDownList);
            cmd = new SqlCommand("select * from Assign_to", con);
            sda = new SqlDataAdapter(cmd);
            dt = new DataTable();
            sda.Fill(dt);
            DropDownList2.DataSource = dt;
            DropDownList2.DataTextField = "STATUS_NAME";
            DropDownList2.DataValueField = "STATUS_NAME";
            DropDownList2.DataBind();
            DropDownList2.Items.Insert(0, new ListItem("--SELECT_NAME--", "0"));
           
        }
    }

    protected void Update(object sender, EventArgs e)
    {
        GridViewRow row = (sender as Button).NamingContainer as GridViewRow;
        int COMP_ID = Convert.ToInt32(GridView1.DataKeys[Convert.ToInt32((sender as Button).CommandArgument)].Values[0]);
        string STATUS_ID = (row.FindControl("DropDownList1") as DropDownList).SelectedItem.Text;
        string ATTEND_BY = (row.FindControl("DropDownList2") as DropDownList).SelectedItem.Text;
        // string remark = (row.FindControl("TextBox5") as TextBox).Text;
        string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
        using (SqlConnection con = new SqlConnection(constr))
        {
            using (SqlCommand cmd = new SqlCommand("UPDATE comp_box SET STATUS_ID = @STATUS_ID, ATTEND_BY = @ATTEND_BY WHERE COMP_ID = @COMP_ID"))
            {
                cmd.Parameters.AddWithValue("@STATUS_ID", STATUS_ID);
                cmd.Parameters.AddWithValue("@ATTEND_BY", ATTEND_BY);
                cmd.Parameters.AddWithValue("@COMP_ID", COMP_ID);
               // cmd.Parameters.AddWithValue("@REMARKS", remark);
                cmd.Connection = con;
                con.Open();
                cmd.ExecuteNonQuery();
                con.Close();
            }
        }

        DropDownList2.Items[1].Attributes.Add("disabled", "disabled");
        DropDownList2.Items[2].Attributes.Add("disabled", "disabled");           
       
    }
}