>To specify a little bit further, with option 1, the user can still open the table (with Excel for example) and view the content. You will need option 2 or a different backend (like SQL Server) to prevent a client to view the content of a table.

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>1) Have all data access go through a DLL. The user won't have direct access to the data.
>>2) Use an encryption tool. Look at www.hallogram.com for one.

Not necessarily. The DLL can log onto the server using a different username. In this manner, you can give rights to the DLL, but not the user.