How can I prevent a framepage from being viewed outside of its frameset ?

Answer:

Insert this javascript in the head tags of all the framepages you don't want to be viewed out of the frameset.

<script language="javascript">
<!--
if (top == self) self.location.href = "NAME OF MAIN FRAMESET PAGE";
//-->
</script>

Just set your frameset pages with this code and when a framepage is loaded in the browser while not part of a frameset it will redirect to your main frameset page or the page you specify.

There are also some dedicated extensions that might help you available on the Macromedia Exchange: