If you want to disable the back button of the browser put this code in the body section of your page.
<script type="text/javascript">
window.history.forward();
</script>
If you are using Master Page, then place it in Master Page's body section. It will apply to all the pages which are using the Master Page. Otherwise you need to put this in every page of your website, If you want to disable back button for every page.
Thanks,
Virendra Dugar