В sshd_config:
Match User someuser
ChrootDirectory /some/directory
Так же, в "man sshd_config" сказано:
The ChrootDirectory must contain the necessary files and directories to support the user's session. For an interactive session this requires at least a shell, typically sh(1), and basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4), and tty(4) devices. For file transfer sessions using ``sftp'', no additional configuration of the environment is necessary if the in-process sftp server is used, though sessions which use logging may require /dev/log inside the chroot directory on some operating systems (see sftp-server(8) for details).