Well first of all, I must write that this activity is not recommended by professionals. So first of all try to avoid such alternates and try to focus on better solution.

However with the intentions to share knowledge and skill ( and perhaps you have had no other option besides meeting stupid client requests ), i asm providing a javascript that will disalbe the browser back button. The button itself won't be disabled, it would be not functional. So here we go :)

Call it from desired page, add to the page's onload client event ( where you want to disable browser back navigational button)

<body onload="changeHashOnLoad();">

cheers, you are go on IE9 and Firefox 5 (my test browsers;) )