Road users are occasionally caught without licences. Mr. Hempels situation is not much different from a car driver. Since aircraft are very expensive and flown by multiple pilots, "wheel clamping" is not a practical option.
It "may" be possible to detect an offence by detecting an aircraft associated with a particular person, but you can't pull over an aircraft.
There is/was a well-known pilot known to operate around Benambra (VIC) having legend status due to some of his daring feats (and record number of licence suspensions). He was allegedly flying his Cessna on a charter with a plaster cast on his leg, having broken it in a bad parachute landing. DoA inspectors suspended his licence based on information supplied by the local copper who was "out to get him".
The point I'm trying to make is that DoA and now CASA, have been known to target, prosecute and suspend pilots they believed were breaking the rules. In Hempels case they had all the necessary evidence but seemed to lack the will and discipline to properly enforce the regulations, or applied the rules selectively and with no expense spared viz the Quadrio prosecution.
You have to wonder whether CASA have "mates" who can get away with breaking the rules, while others get worked over for minor technical infractions.