ReadyNAS102 - protection of truecrypt-file against delate

All my data on RN102 are saved in one truecrypt-file -"truecrypt_dad"- created on a laptop (OS Debian 8). This means that the RN102 manages only one file -"truecrypt_dad"-.

With the below described permission structure both users "dad" and "mom" can read and write the file "truecrypt_dad" from the laptop (OS Debian 8).

I want to protect the file "truecrypt_dad" against delete from user "mom", but I cannot find a solution.

MY PROBLEM: The user "mom", by attempting to delete "truecrypt_dad" does receive an errore message - "permission denied - you have not sufficient permissions to move the file into the trash" (free translated from german) - which is OK for me. But on the same window the button "DELETE" is still active and the user "mom" can delete the file!. To complete the confusion the user "mom" cannot rename the file due to insufficient permissions, which is also OK for me.

On one side the user "mom" cannot rename (OK for me) and cannot move into trash (OK for me), but on the other side can delete (not OK for me) the file.

I want to protect the file "truecrypt_dad" against delete from user "mom". There is a solution and an explanation for the strange behaviour mentioned above?

Thank you.

Technical data

Laptop description: OS Debain 8

extract from /etc/fstab of user "dad"

#nas

//IP/dad/ /media/dad cifs users,noauto,username=dad,passwd=dad 0 0

extract from /etc/fstab of user "mom"

#nas

//IP/dad/ /media/dad cifs users,noauto,username=mom,passwd=mom 0 0

NAS description: ReadyNAS102, Firmware 6.6.0, 2x 2TB

Configuration after factory reset:

SSH: activated

USERS/GROUPS

User     Group    comment

admin    admin    default user

dad      parents

mom      parents

SHARES

file/directory           owner  group    permissions   comment

/data                    root   root     drwxr-xr-x    default share; default permissions

/data/dad                dad    parents  drwxrwx--T+   ACLs and Sticky-bit created by RN102; ALSs: rwx for admin

/data/dad/truecrypt_dad  dad    parents  -rwxrwx---+   file created with tuecrypt on laptop