You can map an SMB username onto the Unix user ``nobody''. Since SMB passwords are stored separate from Unix passwords, you don't have to worry about people getting access to the machine.
Give /etc/samba/smbusers a line like this:
nobody = private
And give the user nobody a Samba password:
smbpasswd -a nobody
For the share itself, share it like this:
writable = yes
browseable = no
public = no
valid users = nobody
path = /path/to/share
Now you should be able to access the share with the username private and the password you gave to nobody.
If this doesn't fit what you wanted to do, just say so. I can help, so just ask away.