Welcome to the club.
The answer is no, permissions on this box is an all or nothing affair. This is one of the complaints that is floating around about the current firmware.
This is odd since the Product data sheet says that you can set quotas for different users. If you can set different quotas why not be able to set different permissions.
SIMPLIFIED MANAGEMENT
With the included feature-rich backup software, you
can create schedules and rules including real-time full
or incremental backups. The Web-based configuration
software allows users to establish storage quotas
for specific users and groups. The DNS-321 can
automatically back up your data regardless of the
Operating System (OS) of your computer (Windows®,
Mac®, or Linux®).