What version of FreeBSD? If 7.2-RELEASE or later, you have the ChrootDirectory
directive available: sshd_config(5)
If an earlier FreeBSD version, you can run a more current sshd (with this feature) from ports: security/openssh-portable
That will keep them in a particular directory.
To hide their processes from one another, (as root) run:
# echo 'security.bsd.see_other_uids=0' >> /etc/sysctl.conf
# /etc/rc.d/sysctl reload