openssh: Enabling sftp, but disabling ssh?

Does anybody know if it's possible, using openssh, to allow file
transfer to/from a machine, using sftp, for a specific userid, and
disallow ssh login/remote command execution for that same userid? Other
userids on the machine should be unaffected.

We would like to create a user which has a home directory and r/w access
to a specific dropbox directory (possibly event make the dropbox the
home directory for the user). All attempts to play with /etc/passwd and
retricted bash have been in vain, and I haven't been able to see any
config options in sshd_config for it. No luck googling on it either--it
does appear others have tried this and in some cases have some kind of
hacked-up setup (partially?) working. I'm looking for a more standard
solution.

Any help is greatly appreciated...

Thanks,
Mark