Can't believe that I'm +1 to this 3 YEARS after the request. We've implemented Advanced Folders since then and still not found a way to do this. In fact, we have made it HARDER because, as an owner of an advanced folder, you cannot make a folder read-only on any device you claim ownership of!
+1 This is an essential feature. No one wants read/write on their backup servers. I've paid for PRO and was very disappointed when I could not read-only the shares on my backup server once it was linked to my identity. Seems to me like a pretty big oversight in the massive 2.2 upgrade. Guess I'll have to go back to 1.4 folders, unlink my devices and link everything manually (and not use any of the features that I paid for...). Please address this!