Key Pad

How do I use only 1 Key pad code on my Ford f150?
I have been using my factory set key pad code and recently changed it to a personal code, however the factory set code works as well. How do i disable the factory set code and only use my personal code? A person I knew before knows my factory code and has been stealing things from my vehicle and I would like to prevent this!
The factory module has the code in it. Its called a GEM module, (general electric module). The dealer cannot change the factory code. You have to purchase a new GEM module with a new code.
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