"Mandate monthly electronic filing of it and if it shows a trend of flying more than working, say as a plumber, then pay the bloke a visit and check his bank account. In addition there could be a record of where they have been, whether it was night or day and weather conditions at the relevant times. No cost to the regulator, plenty of it to the likely candidate"
Cost to EVERY PPL! Cost to the pilot of an Evans VP2, a Turbulent, or a Fournier.
Airport records should already be sufficient for detecting the likely aircraft types.
Caveat emptor would be better.