Cancel a cross-page postback? - asp.net

Question

I have a page that has several ListBoxes that have some cascading filtering based on the selected values using an AutoPostBack. The form takes all the selected values and generates an excel doc by cross-page posting to a different ASPX. The problem is, after clicking submit once, it will continually fire the cross-page postback every time a selection has changed.
<asp:ScriptManager runat="server" />
<asp:UpdatePanel UpdateMode="Conditional" runat="server">
<ContentTemplate>
<asp:ListBox ID="ParentItems" runat="server" SelectionMode="Multiple" AutoPostBack="true"></asp:ListBox>
<asp:ListBox ID="ChildItems" runat="server" SelectionMode="Multiple" AutoPostBack="true"></asp:ListBox>
</ContentTemplate>
</asp:UpdatePanel>
<asp:Button ID="Submit" runat="server" PostBackUrl="~/AnotherPageThatGeneratesAnExcelDoc.aspx" />
How do I cancel the cross-page postback from the ListBoxes' SelectedIndexChanged events?
Here's the event in the codebehind:
Protected Sub ParentItems_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles ParentItems.SelectedIndexChanged
'' do some filtering of the ChildItems ListBox
'' tried these but they do not work
''Submit.Enabled = False
''Submit.PostBackUrl = String.Empty
'' I also tried wrapping the button in a PlaceHolder and hiding/removing it, neither worked
''Buttons.Visible = False
''Buttons.Controls.Remove(Submit)
End Sub

Answer 1

This is my current solution using javascript. It works, but seems like a hack:
// using jQuery, add a click event that resets the form action
$("select[multiple]").click(function () {
this.form.action = this.form._initialAction;
});
Edit: adding a click event in the codebehind:
ParentItems.Attributes("onclick") = "this.form.action = this.form._initialAction;"

Answer 2

The problem is that using the PostbackUrl property resets the form action to a new URL, and your Ajax calls (or any subsequent postbacks) use whatever the current action of the form is.
Your solution doesn't work because the submit button isn't part of your UpdatePanel, so it never gets modified.
The easiest solution might be to move your Excel file generating code out of the page it's in, and into the page you're looking at, in the click handler of the button.
You also could probably include an iframe on the page you're looking at, and on submit, rather than going to a new page, set the source of the iframe to the Excel-generating page.
Both of these would avoid the need for using the PostbackUrl.