Use onkeydown event
<asp:DropDownList ID="DropDownList1" runat="server" onkeydown="return false;">
<asp:ListItem>a</asp:ListItem>
<asp:ListItem>b</asp:ListItem>
<asp:ListItem>c</asp:ListItem>
</asp:DropDownList>
in C# windows Application
private void comboBox1_KeyDown(object sender, KeyEventArgs e)
{
e.Handled = true;
}