No user should ever have access to the Navigation Pane so you need to shut that down also. All employees should also be given a firm warning that attempting to crack the application and/or modifying any objects in the database is a firing offense. Then you lock the FE down as best you can and there are several techniques where you can bury the BE so that the users can't see it to link to it from their own FE.
@isladogs has written extensively about securing your databases. But, in the end, Access databases are never truly secure.