Some smart guy said this originally; I'd shake his hand if I could.
I think an expiration date on a c-card might do it. Maybe if you were a basic diver, your card would expire in 5 years unless you could show (and tell) that you had dove 25 times, or something. Anyone can BS a logbook entry.
Obviously, further training issues a new card.
Maybe higher up levels of training would have longer durations before expiration.
Maybe Scuba organizations could have a quickie refresher course which issued a new card on the beginner level..
Something simple that works.