<asp:DropDownList ID="oDDL" runat="server" Width="218px">
  <asp:ListItem Value="1">Путин</asp:ListItem>
  <asp:ListItem Value="2">Владимир</asp:ListItem>
  <asp:ListItem Value="3" Selected="True">Владимирович</asp:ListItem>
   <asp:ListItem></asp:ListItem>
</asp:DropDownList>

<input type="button" id="oChange" onclick = "DoSome()" value="OK" />

<script type="text/javascript" language="javascript">
<!--
function DoSome()
{
	var DdlID = '<%=this.oDDL.ClientID %>'
	var elem = document.getElementById(DdlID);
	elem.selectedIndex =  0 ;
	elem.disabled = true;
}
</script >