|Not exactly sure what you're trying to do from your question, but I'll make a guess...|
Sounds like you have POS terminal(s) accessing database on back-office server and want to limit database access to POS terminals only. If this is true, you'll need to:
Turn off simple file sharing on the system acting as the server.
Create userids on the server for each POS terminal using the same userid/password combination used to logon to the terminal.
Apply read/write permissions to the folder containing the database file(s) for ONLY the new POS terminal userids.
Local access can be secured in a number of ways. The one I'd recommend is to simply make sure the server is physically secure. That is, limit access to the computer itself if possible. At least, use a secure password for Windows and make sure you log off when it isn't being used.