Locking low-access usr to directory?

I'm doing an assignment setting up a small ISP setup on a FreeBSD machine. We are using Unix login accounts for users to access their host space.

1 - I create users using adduser -v
2 - I add users to the group www-usr
3 - In the /usr/home/<user> dir, I add the directories www, mail and log

How do I lock the user to /usr/home/<user>? I don't want them going further up the tree than here when they use sftp to transfer files in.

Any help appreciated!

One way would be to use FreeBSD jails. Google to find out more about FreeBSD jails.

thanks for the reply but... the simulation already runs inside a jail :S it's possible to make a jail-in-a-jail, yeah? but that might be more complicated than a permissions based approach