Match User sftponly
AllowTcpForwarding no
X11Forwarding no
ForceCommand /usr/libexec/sftp-server -l INFO
Replace sftponly with your short user name, then save the file and quit the editor.
[robg adds: You'll probably have to restart Remote Login in the Sharing panel to make these changes take effect, but I'm not sure of that, as I haven't tested this hint. It's categorized as an OS X Server hint, but I have no reason to think it wouldn't work in Client as well.]

