After logging off, a user is able to see the pages by pressing back button of the browser
This is a result of the browser caching the page, there's not a lot you can do about it really.

Some browsers might respond to setting cache control headers like the following:

header("Cache-Control: no-cache, must-revalidate"); // HTTP/1.1
header("Expires: Sat, 26 Jul 1997 05:00:00 GMT"); // Date in the past


which might force the browser to re-load the page regardless of it's local cache.

There are apparently some options available to control this in Safari specifically, but I haven't been able to find anything for other browsers. I imagine the same would work for Chrome (being Webkit based), but I can't comment on the the other major browsers.