Which distro are you using?
Sftp is part of OpenSSH. If you turn of the ssh daemon (service), you
can turn of sftp. This is easy for me in Slackware: I would just
remove the executable permission from rc.sshd:
chmod -x /etc/rc.d/rc.sshd
You probably have System V init instead, so snoop around in
/etc/init.d. When I say "snoop" I mean do it as a regular user so you
can avoid accidentally deleting something.