I have a spreadsheet that is used by several users and the users can
be added dynamically by an admin. We do not want users to be able to
see each other's data so when a user opens the file he is prompted for
a password that is stored on an xlSheetVeryHidden sheet and is only
allowed to see his own sheet. There are other controls in place to
prevent the user from editing any sheets or macros.

The problem is that a very smart user could see another user's data by
disabling macros when opening and then (assuming he knows the name of
the other user's sheet which would not be hard to figure out) type in
a formula like:
=OtherUser!A1

and be able to see the other user's data. Any way to prevent this?
Note that I cannot just password protect the workbook because they all
need access to the same workbook. If I password protect the sheet that
opens with macros disabled he can just insert another sheet and put
the formula there.