Disable sharing from desktop ?

We are using syncplicity for Documentum to share folders/documents in our company.
Folders are synced from documentum with a "reader" access. That's great.

But users can still share a folder (already shared from documentum, or a new folder) with others people, and nothing in policies available can restrict that (we can only restrict sharing to other company users).

We would like to disable sharing for users, excepted from inside Documentum (where only a specific group has the right to do it).

How can we do that ?