You still can put a big red sign with "This browser is not supported, here's why.", and let the user decide if they want to try it with IE just to see if is worth it the browser change.
They are going to fail either way - it's better to fail gracefully, no?
Their problem is that they want to be "pixel-perfect" across all browser. I get why, but as a User I don't really care as long as everything else works. And chance are that with IE9 and IE10 it will.
I'm not a IE user, but when we stop supporting IE6 we put a big sign saying that if there were problems we won't fix them and to download a modern browser. But people still went to the site even if some divs were in crazy places because basic function they needed worked and maybe they hadn't the chance of upgrading their browser.
If one of your client is once on another computer with only IE installed and doesn't have permission to install another browser, the fact that he can't use the website at all, is that a feature ?
https://github.com/h5bp/html5-boilerplate/blob/master/js/plu...