Trying to restrict a folder within a directory created in linux filesystem. I have changed the permissions to: root rwx, a special active directory group rwx and all others r. Upon doing so, people that are not in the special AD group can access the directory and modify files. Upon doing so the group changes to "Domain Users" when the user modifies documents within the directory. I have to manualy change the documents default group back to my AD group. I have tried to create another AD group and modify permissons to deny write access. When doing so through windows explorer, the settings seem to take affect until I go back in a look at permissions for the restricted group. No permissions show when I view for the second time. Please assist.

Samba share properties

[MyShare]
comment = "blah blah blah"
browseable = yes
guest ok = no 
read only = no 
path = /xxx/xxxxx/ 
create mask = 0640 
directory mask = 0750 
admin users = @"domain\Domain Admins", @"domain\group A", @"domain\group B" 
valid users = @"domain\Domain Admins", @"domain\group A", @"domain\group B" 
nt acl support = Yes 
inherit acls = yes 
inherit owner = yes 
inherit permissions = yes