The browser back button cannot be disabled as the browser security will not allow this. If you want user to stay on the same page, even if user presses back button, try this java script..
<script language="JavaScript">
javascript:window.history.forward(1);
</script>
What if the JavaScript is disabled by the client, then this code doesn't work. And also it works great in IE, but not in other browsers. So I do not highly recommend using this.
Here is another solution for back button problem..
Add this code to Page_load
Response.Cache.SetCacheability(HttpCacheability.NoCache);
Response.Cache.SetExpires(DateTime.Now);
The above code will disable page cache. Since the page is not being cached on the browser, the page will be reloaded when the user hits the back button.