Hello Cal,

If your slides don't have animation, you could File > Save As a PNG or JPG,
then insert the resulting images into *your* slides.

Otherwise, you could create Slide Masters (View > Master > Slide Master) for
your content. That way, instead of editing on the actual presentation slides,
the content would have to be edited on the master. It's not full proof -- I
guess it depends on the skill of the other group members (can/will they
access the masters?)

Cal,
Not really. There is this though, something designed for larges compagnies
(IRM - Information rights management):
Office 2003
http://office.microsoft.com/en-us/help/HP062208591033.aspx
Office 2007
http://search.microsoft.com/results.aspx?mkt=en-US&form=MSHOME&setlang=en-US&q=i
nformation+rights+management
Also see here as to the protection of PPT presentations:
http://pptfaq.com/FAQ00038.htm
Luc Sanders
MVP - PowerPoint

Unless IRM is a possibilty there's no real foolproof way to do this but this
will fool most of the people most of the time.

Start with a totally blank slide. Zoom out to 10%. Create a rectangle as
large as possible and set its fill to 99% transparent. Make two or three
copies and then ctrl A > ctrl C and  then use ctrl V to paste over any slide
you want to "lock"