You might struggle there, as part of the underlying philosophy of Radarr is that it is in control of your library. Because it's primary function is to put files into your library as it finds them, write access is assumed.
Are you doing this as you have a share that you want it to be aware of, but do not want it to change (that is, you've got the files as you want them)? If so, you might have to set it to set it to Read/Write and import the titles as "Not Monitored". I've got mine set up like that for one share, but can't remember how I did it. Somebody else may better answer this, but I've got a feeling that when you import titles into Radarr using the bulk import facility, all titles are set to "Not Monitored". That way, Radarr will never touch them unless you ask it to manually.
As a side-note, you'll need to mount your "/data" as RW/Slave, if you haven't already.