Years ago I had a similer situation, I sold a diagnostic
tester of a very simple circuit design. one small single
sided PC board with a socketed micro. The circuit was
very simple all the "smarts" was in the micro. These were sold
in an industry where most all our customers had device
programmers and therefore could easily copy the circuit and
program there own device. What I did was during assembly of
the PC board I added a very thin strand of wire from an
unused port pin to the ground pin, These pins were next to each other.
When the socket was soldered to the PC board the
wire was hidden under the socket. I even included a small
copy of the schematic inside the case if anyone ever opened it
this showed the pin with the hidden jumper as NC (no connection)
Lastly in the code at boot up It checked for a ground on this
pin if none was found the Micro went into an endless loop
and did not function. This can also be done if you have 2
adjacent pins you do not use set one low read the other pin
see if it is low. Just keep the jumper hidden.

Bruce