Re: Leasing a new plane
"how does one enforce such a restriction"
Ultimately you have to trust the instructor. The trouble with GA is that there are soooo few people that one can trust (as you will rapidly discover). Poor cashflow goes hand in hand with such personal characteristics, and debt collection is normally a substantial challenge. I could post a few shocking stories involving some well known maintenance businesses (paying only on the bailiff's second and final visit) but of course won't.
You do have to trust people. You have to trust them to manage the engine correctly (most instructors, even IR types, have their own weird ideas on this). You have to trust them to not tamper with the fuel flowmeter FOB setting (very tempting, I got this). Fiddling the airborne times is pretty routine. You end up downloading the EDM700 engine logs and measuring off the airborne times and checking them against the declared times. Etc.