Hi,
your code does exactly what it should like ;).
you are inserting a new ListItem with an empty string at INDEX 0 of your dropdownlist.
This should work:
ddlRepUserCreated.Items.Add("Please choose a user");
Anyway I would remove your
<asp:ListItem>Please choose a user</asp:ListItem>
</asp:DropDownList>
and add it through code (see obove items.Add("")) after retrieving Data from SQL.
EDIT: AppendDataBoundItems="true" is also missing in your Dropdownlist Properties.
Regards