make drop down list item unselectable

Not sure if you are still looking for an answer for this?

Mark Redman's answer is great if you can define the select list in the aspx page, however if you bind the drop down list dynamically obviously you cannot.

I had success using the following to achieve the result you are after (not sure on full browser support but works in newer versions of IE)

foreach ( ListItem item in dropdownlist.Items )
{
    if ( [item should be disabled condition] )
    {
        item.Attributes.Add( "disabled", "disabled" );
    }
}

This will render your disabled elements greyed out.

You can disable an <option> tag in an html <select>

See: http://www.htmlref.com/reference/appa/tag_option.htm

in asp.net:

<asp:DropDownList ID="MyDropDownList" runat="server">
        <asp:ListItem Text="Standard Report" Value="SR"></asp:ListItem>
        <asp:ListItem Text="Enterprise Report" Value="ER" disabled="disabled"></asp:ListItem>
    </asp:DropDownList>