Page Redirection Prevention

I was wondering if anyone knows a way to prevent page redirection? I would like to capture an event when the page leaves, and then run some code. If the code behind meets certain criteria, enable redirection, otherwise cancel it. I know that some of them can be accomplished using Javacript, but the problem I came across actually stops the redirect. Here is what I have:

JavaScript:

<script type="text/javascript">
   window.onbeforeunload = confirmExit;

   function clickButton(getId) {
            var getBtn = document.getElementById(getId)
            getBtn.click()
        } 

   function confirmExit(e) {
       var btnId = document.getElementById("<%=btnConfirmExit.ClientId %>").id
       clickButton(btnId)
    }
 </script>  

Code behind:

Protected Sub btnConfirmExit_click(ByVal sender As Object, ByVal e As EventArgs) Handles btnConfirmExit.Click
    'if something, cancel request

    'else
     ' let it go
end Sub

If you tried to use:

Page.Response.End()

But it does nothing. Does anyone have any ideas?

thank