If you open up a JS window, you don't have to allow a back button... but thats about the best you can do.
Though, what you could maybe do, would be to display nothing on a particular page if the HTTP_REFERER is the page that it sends you to?
Ie - Thats assume its a voting page...
www.mysite.com/vote.php
when you vote you get sent to www.mysite.com/votethanks.php
So for vote.php, add a HTTP_REFERER check in, if its equal to the URL for the votethanks.php page, exit;
Just an idea?