Do you want "security against the
malicious" or "guidance for the
cooperative".
Only by setting up full blown user level
security (advanced topic) can you come
anywhere close to the former (and that is
just coming close). Putting passwords into
the Open event of certain forms clearly
falls into the latter category and that is
only possoible with forms, not with tables
or queries.
In the latter category there are a handful
of obfuscation techniques that are easy,
painless, and at least force "snoops" to be
fairly familiar with Access.
So, what sort of "blockage" did you have in
mind?