I'm sorry but there's no e.g. official "AnnonSFTP"-Patch/Modification for OpenSSH. As far as I know you're not able to splitt the SFTP from the SSH-Account (I don't mention any unofficial Patchs wich may work).
Hi, Just in case you don't know, "scponly" works great. In our datacenter we need to give users access to "ftp" but we also need a secure access. Since the users are not allowed to gain SSH access we use the "scponly" solution. So far it has been working great both with GNU/Linux users and with Windows users. There are several clients which support both SCP and SFTP. The scponly can be found here http://www.sublimation.org/scponly/ Kind regards, Rico