The network shares ("exports") available on a NFS server can be listed with showmount -e myserver. It will list all exported directories, even if the client is not allowed to mount all of them.
This here is a workaround. A real solution - hiding all exports that the client does not have access to, but showing the rest - does not exist to my knowledge.
With NFS v4 it is possible to limit listing the shares to select clients only: By restricting access to rpc.mountd via /etc/hosts.allow:
mountd: client1 client2
This will allow client1 and client2 to list the shares:
root@client1:~> showmount -e myserver Export list for myserver: /data/media client1,client2 /data/backup client1,client2,client3
On client3 it will lead to an error:
root@client3:~> showmount -e myserver rpc mount export: RPC: Authentication error; why = Failed (unspecified error)
The downside is that NFS v2 and v3 mounts will not work anymore, because a mount command in this versions needs to contact rpc.mountd.