Added... 
You can configure the DNS-327L to restrict access to specific directories using group and user policies
Yes. That is correct and the access restrictions do work.
The goal however should be that when a user with restricted access logs into his share he shouldn't be able to move up in the directory tree and browse (view not enter) all the other users shares and also Volume_1.
If i am not wrong in windows systems this behavior can be controlled with the policy setting "bypass traverse checking".
There are other NAS brands that allow "advanced share privileges" as:
-disable directory browsing
-disable modification of existing files
-disable file downloading
but i am not sure if this is what we are looking for.
Another useful addition would be the possibility to set quotas for a shares.
I know we can set quotas for users and groups but if for instance i have a Public share with all accounts access rights and want to limit it's size i don't know how to do it without a share quota setting.
Thanks