I assume the issue is the scanner has no user account assigned to it so as soon as you apply share or NTFS restrictions the scanner can no longer write to the share. Does the folder actually have to be shared? If not, it is a little convoluted, but you could save it to a local folder on the PC (not shared) and use syncing software such as
www.secondcopy.com to sync it with a server share that has file restrictions. For added security disable file sharing on the PC and no one will be able to access even as a hidden folder.