I think the better solution is that you change your code to something like this:
dropDownHeight = (
)(Annees.Buttons.Count * 25);
(dropDownHeight > 250)
// set drop-down height, scroll bar is shown
Annees.DropDownHeight = Unit.Pixel(250);
// don't set drop-down height, no scroll bar will be shown
Annees.DropDownHeight = Unit.Empty;
Princy's solution will hide the scrollbar even when you need it to be there.
I hope that helps.