ComboBox, the name itself describes that it is the combination of both DropDown and TextBox. So I don't think there would be any way to make it non-editable at all.
Or you may go for enforce not to write anything into box on keypress, by handling the keypress event to do nothing or to avoid type it in combobox.
Ex :-
private void cmbTest_KeyPress(object sender, KeyPressEventArgs e)
{
e.Handled = true;
}