http://www.sublimation.org/scponly/
Use 'chsh(1)' to set someone's login shell to be scponly, and then that user can use scp but not get an interactive shell. You may also want to put them in a chroot(8) jail to prevent them snooping around your system (see ChrootNotes).
Your package management probably has this - Debian users can 'apt-get install scponly'.
See Also:
2 pages link to SCPOnly: