[C# net4] DropDownList disabled after change value

Hello guys, hope in your appreciated help.

It possible disable one DropDownList after changed the value in the same DropDownList?

How to intercept the change in DropDownList ?

my problem is:

I've one dropdownlist with or without selected value:
[CODE]





private void txtDDL()
{
txt.Items.Clear();
txt.Items.Add(new ListItem("------", ""));
txt.Items.Add(new ListItem("A", "A"));
txt.Items.Add(new ListItem("B", "B"));
txt.Items.Add(new ListItem("C", "C"));
txt.Items.Add(new ListItem("D", "D"));
txt.AppendDataBoundItems = true;

mySQL = "SELECT txt FROM dotable WHERE txt = ? ";

OdbcCommand objCmd =
new OdbcCommand(mySQL, myConnectionString);
objCmd.Parameters.AddWithValue("param1", dbDDL.SelectedItem.Value);
objCmd.CommandType = CommandType.Text;
objCmd.CommandText = mySQL;

try
{
myConnectionString.Open();

OdbcDataReader dr = objCmd.ExecuteReader();
while (dr.Read())
{
if (dr["txt"].ToString() != "")
{
txt.DataTextField = "txt";
txt.DataValueField = "txt";
txt.DataBind();
txt.SelectedValue = dr["txt"].ToString();

if (dr["txt"].ToString() == "A" || dr["txt"].ToString() == "B")
{
txt.Enabled = false;
}
}
}
}
catch (Exception ex)
{
throw new Exception(ex.Message);
}
finally
{
myConnectionString.Close();
}
}[/CODE]
when I select one new value of my txtDDL fired this event:

[CODE]protected void Save_SelectedIndexChanged(object sender, EventArgs e)
{
OdbcCommand cmd = new OdbcCommand("update ....", myConnectionString);
cmd.Parameters.AddWithValue("param1", txtDDL.SelectedItem.Value);
cmd.Parameters.AddWithValue("param2", DateTime.Now.ToString("yyyy-MM-dd hh:mm:ss"));
try
{
myConnectionString.Open();
cmd.ExecuteNonQuery();
cmd.Dispose();

StringBuilder sb = new StringBuilder();
sb.Append("");
Page.ClientScript.RegisterClientScriptBlock(this.GetType(), "MyScript", sb.ToString(), false);

}
finally
{
myConnectionString.Close();
}
}[/CODE]
when the event is fired and return to home page I need disable the dropDown List.

Any suggestion would be greatly appreciated
Thank you in advance