Prevent user from choosing Default Item in the ASP.Net DropDownList

I would suggest simply make use a Validator to validate the Default Item if you don't want to insert the default item

    <asp:DropDownList ID="DropDownList1" runat="server">  
            <asp:ListItem Text = "Select one" Value = "0"></asp:ListItem>  
            <asp:ListItem Text = "One" Value = "1">One</asp:ListItem>  
        </asp:DropDownList>  
        <asp:RequiredFieldValidator  
             ID="RequiredFieldValidator1"  
             runat="server"  
             ControlToValidate="DropDownList1"  
             InitialValue="0"  
             ErrorMessage="This field is required"  
             >  
        </asp:RequiredFieldValidator>  
        <br />  
        <asp:Button ID="Button1" runat="server" Text="Button"/>