How can I stop a user from viewing pages visited by the previous person?

Hello. Just to warn you this is my first posting to the forum.
As you can see from the title I have a problem. Let me explain.

I am developing a web site for a group of retailers. This will include news, training and a forum.etc. The site will offer two protected areas. One for the shop staff, and the other for the owners.
I have purchased a php security package to enable me to give them a username and passwords to the various areas. This all works fine. BUT the pages viewed are still available from the browser history button. This is ok if the owner has been viewing at home. But if they had used a pc at there shop a member of staff could then access the pages without needing the log in details. This could be information not to be seen by the staff.

I have looked via the search engines at how to stop the pages being cached and have found this code.
<META HTTP-EQUIV="PRAGMA" CONTENT="NO-CACHE">
<META HTTP-EQUIV="Expires" CONTENT="-1">

I have tried it both in an html and php page but it does not seem to make any difference I can still view the page from history when I reopen the browser.

I have seen on some sites that when you go to history when not connected to the Internet the viewed pages are greyed out meaning that you have to reconnect to view them. This would be fine since the security software would require the password etc before letting the person view the page.
That is the background but does anyone know how I can achieve this or my site. In simple terms please I am new to a lot of this.

Thanks Kevin.

Protect the pages by sessions. You have of course to ensure that the user closes the browser window / or log out, but that should do the trick.