Paul provided the correct answer - "You can't." Some things you can do,
though, is present your web page as a graphic instead of HTML, but then users
can always print the graphic and it's much more difficult for you to update
your web page(s). You could also convert your web page(s) to Flash, but
again, users could still take a screen capture of the information. Bottom
line is if you post it on the Worldwide Web then it's universally accessible
(i.e., the reason for the WWW).