So, let me get this straight - do you steal the tech log after the flight in case you don't get paid, or break into the aircraft and steal it after you haven't been paid.
That's a great idea, you probably wouldn't even get a custodial sentence if it was your first offence. But even if you did get a criminal record and a little time in prison, at least you'd have a moral victory right?
If you are a freelance pilot, you in effect run your own business, credit control is part of running a business, I don't think theft and burglary are in the "how to run a business" manuals I have read.
Why not do it proffessionally - draw up a booking form for the operator to sign when the book your services - this can include payment terms and cancellation fees. Include in there your expectations for payment of your day rate and your expenses.
If you are owed money it is a contractual matter - so why take a booking without a contract, then you have the law on your side.
Phil