Personally, I don't think it's a great idea to disable the backspace key, or the back button of the browser. Instead, you should code around it, so your app behaves normally, even when users browse back and forth in your site.
If you do insist, take a look here:
http://www.4guysfromrolla.com/webtech/111500-1.shtml for possible solutions.
Cheers,
Imar
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Imar Spaanjaars
Everyone is unique, except for me.