Multi-User Security

[David E. Meier]

Hello list.

I would like to get your opinion on what is a safe multi-user environment.
The scenario:

We would like to offer to some customers of ours some sort of network
backup/archive. They would put daily or weekly backups from their local
machine on our server using rsync and SSH. Therefore, they all have a user
account on our server. However, we must ensure that they would absolutely
not be able to access any data of each other at all.

What is the "best and safest" way to do so? Regular UNIX permission
settings? File system ACL's? User jails? Restricting commands in their
path environment? Or would it even make sense to encrypt the file system?
How would some of the solutions affect data backups/restore on our side?

Any comment on this is welcome. Thanks. Dave.