How to restrict ftp users from getting console access

We have set up an FTP server with several users that also have access to the server through SSH, which is not what we want.
The way we have set up the server is not probable the best one, but it just works for us. We have a Debian server in which we create normal users that we modify their home directory to point to the FTP filesystem, /ftpdata, which is an LVM disc.

So, to remove console access we need to do two things, adding /usr/lib/sftp-server as a shell to the /etc/shells file:

root@host # echo '/usr/lib/stfp-server' >> /etc/shells

and modify the default user's console to sftp-server by editing /etc/passwd or

root@host # usermod -s /usr/lib/sftp-server username