Allow SFTP but disallow SSH?

Starting with version 4.9 OpenSSH (not available in centos 5.x but ChrootDirectory feature was backported) has an internal-sftp subsystem:

Subsystem sftp internal-sftp

And then block other uses:

Match group sftponly
     ChrootDirectory /upload/%u
     X11Forwarding no
     AllowTcpForwarding no
     AllowAgentForwarding no
     ForceCommand internal-sftp

Add your users to the sftponly group. The chroot directory must be owned by root, and cannot be group-writeable, so create a subdirectory for each user, e.g. uploads or home/$username that’s owned by the appropriate user (if you match their home directory, it will be the default working directory when connecting). I’d also set /bin/false as the user’s shell.

As an example, users can then upload single files with:

sftp username@hostname <<< 'put filename.ext uploads/'

(scp will hopefully soon be modified to use sftp so this will become easier)