关闭浏览器时禁止/绕过IE警报消息

I have a web page which has a ajax modal popup being loaded inside it.When the modal popup is there we need to restrict the user from closing the browser.we have handled the onbeforeunload event but it showed a popup "are you sure you want ot leave this page".How to supress this popup and stop the page from closing.I have a web page which has a ajax modal popup