Bottom line, you can't.
If they can see your presentation on their computer screen, then the people
without morals can screen capture and print the capture. If you show it on
a projected screen, they can take a picture of the screen and develop that,
too. To combat this, display a copyright notice (or your countries
equivalent) on the slides. It is not required here in the states, but
serves as a visual reminder that you are serious about it.
You can, however, get some security for someone being able to modify your
presentations. See:
http://www.rdpslides.com/pptfaq/FAQ00038.htm
B
