There is no relationship between FTP and SFTP.
SFTP is a subset of SSH, not FTP, and therefore it's available to anyone who's able to ssh into the server.
You can (and should?) disable the FTP service unless you have some other use for it.
As for SFTP, the question hinges around two things - first, are you running a central directory server, or does each user have a local account on the machine? and second, do the users need shell access on the server for any other purpose?
The reason I ask is that since SFTP runs under SSH, you can use any of the SSH controls to manage access. The first one that comes to mind is
AllowGroups (or, potentially,
AllowUsers).
Using either of these directives will allow you to restrict access to SSH/SFTP to specific users and/or groups.
In that way you could create a group for your users (if you don't already) and then tell SSH that only that group (or list of users) are allowed to connect. For example, if you created a group called 'sshUsers' and put all the users who should be able to SSH or SFTP to the server into that group, you can then add the line:
<pre class=command>AllowGroup sshUsers</pre>
to /etc/sshd_config on your server to restrict access to that group only.
The reason I ask about the central directory is because if you're running a directory, you just need to add the group there. If the server has local accounts, though, then you need to add it in the local directory. It's not a big difference either way, just a matter of where you make the change.
