Found an article that mentions a method of preventing PC CDROM drives (and therefore PC CD copiers) from playing audio CDs by generating error information that an audio CD player will ignore, but will cause a PC CDROM drive to reject the CD as bad:

http://www.esware.net/empire/hardware/cdrom/news/9907/audiolok.htm

Of course (as I think someone else has said), unless you're a hardcore digitial audiophile, you can always rip an incoming analog stream from an audio CD player to get around any conceivable protection.

Gary