If you don't give them the master password then they can't alter the
structure. You should also include an "About" layout. If you use the
Developer it can put its script wherever the About menu lives.
It can be very simple. Here is an example of mine (kind of old). Be
advised, this is not legal advice. This is just what it says to do in
the 5.0 Developer Edition manual. It is required for distributing
"bound" solutions, but I believe it would apply to any solution:
Fenton Jones Consulting
1728 Meade Ave
San Diego, CA 92116-3930
For technical support call (619) 692-1529
Warning: This solution contains password(s) which can only be provided
by the developer identified above.
Portions of this software are copyright 1984-2001 FileMaker, Inc.
All rights reserved.
Copyright 2001 Fenton Jones
(this is optional, but I like it :-)
This notice lets FileMaker Inc. know that you have rights, and they
will not break it open to reveal the master password unless the person
asking can prove they have a license that says so. Explicitly retain
that right only for yourself in a written contract.
This doesn't stop them selling it "as is" to others however. One
simple trick is to hard-code their company name into some of the
layouts. They might even like that, kind of a personalization. Don't
tell them that they can't change it, and that you're doing it because
you suspect they are dishonest.
Tell them they can call you and you'll change it for them anytime for
free. Create it as a field in a 1-record Menu file, which they can't
access, then reference it from everywhere else, so you can change it
quickly, but they can't.