If it's an ext3 directory, then the umask option won't work in fstab. That option is only useful for non-unix filesystems that don't have permissions that linux can use.......sounds like you need to set the permissions through samba........
Don't know if this will help, but for Windows, there is a shareware app called
Folder Guard, which will allow you to set permissions on any FAT32/NTFS filesystem with greater control than NTFS does natively. It allows you to hide files and folder, restrict access to the system, allows users to only access their files, etc.
You might want to read the
faq page for more info