Have you tried searching for your answer?  This is a very common question.

If you distribute MDE files, your users will not be able to open forms,
reports, or modules in Design View. They would not be able to mess
with your code. I also strongly suggest setting limited options under
Tools | Startup. Hide the Database Window, disable the Shift key
bypass, etc. To protect the design of tables and queries your best bet
is to implement full blown Access User Level Security (ULS).
You would then deny all permissions on the tables themselves and create
RWOP queries for the users to access the table information. Assigning
appropriate permissions to various database objects would also be needed.
The users could still possibly import the queries into another container,
but they would essentially be worthless since they would not be able to
access the data in the BE file.

If you have never used ULS before, be prepared for a lot of work ahead
of you. ULS is not a trivial undertaking at all so I would recommend
ALL of the following reading material before beginning. Also, practice
on dummy databases until you are really comfortable with it.

Access User-Level Security:

Security FAQ (the Security Bible):
http://support.microsoft.com/?kbid=207793

Jack Macdonald's Security Document:
http://www.geocities.com/jacksonmacd/AJMAccessSecurity.pdf

Lynn Trapp's Ten Security Steps:
http://www.ltcomputerdesigns.com/Security.htm

Joan Wild's Tips:
http://www.jmwild.com/security02.htm

The Security Whitepaper is also worth reading:
http://support.microsoft.com/?id=148555

Keith Wilby's Instructions (See step-by-step link)
http://www.keithwilby.com/

Other Good Information:
http://www.access-experts.com/default.aspx?selection=TutorialSecurity&sm=18

Other Microsoft KB articles of interest:

Description of how to help protect a Access 2000 database:
http://support.microsoft.com/?id=254372

Description of the role of workgroup information files in Access security:
ACC97: http://support.microsoft.com/?id=303941
ACC2000: http://support.microsoft.com/?id=305541
ACC2002/2003: http://support.microsoft.com/?id=305542

Good luck,