Suggestions:
1) Open all the needed files on the Server end as multiuser
2) Users have access to the files only through Filmaker. (using OPEN =>> HOSTS) That way they cannot get to the actual server (CPU) that is housing the actual FM database.
3) Users do not have the ability to export data. That way the data cannot be extracted either.
Hope that helps. I'm also assuming that all users have their own copy if FMPro software on their desktops