How would it be possible to hide user content from server administrator?

I have been asked by a users of the owncloud 9.1 server I am hosting on my Ubuntu 14.04 machine, whether it is possible to configure owncloud in a way that does not allow the administrator to access their data. As far as I can see owncloud only encrypts data, when it is placed outside of the host server. With default installation I can always read the contents of the data directory as root. Is it possible, to say, have user data encrypted with the user's OC password?

This is discussed here:

Both filesystem level encryption (encryption of the disk by the OS) and file level encryption (encryption of the files by ownCloud) would allow you to access the files. The only way to do this would be to do client-side encryption, which you would need a 3rd-party client for.

Is there such a client? I guess technically, either a desktop app or even the OC browser could do this on the clients computer, right?

Please see