Just for the hell of it, I'd suggest if people really want payment limitation to work, you'd need to give the Justice Department funds for a team of attorneys to prosecute fraud cases. (Now I doubt anyone in Justice has much interest in prosecuting people for violating the U.S.Code in connection with farm payments, even if some conscientious bureaucrat in FSA comes up with some information that warrants referral to Justice. They've got bigger fish to fry.
(This is inspired by a provision in one bill to make farmers ineligible for payments if they're determined to have committed fraud. That's a step, but ask FSA how many farmers fit this description now.)
Absent such provision, payment limitation is like the speed limit.
[note to self--I seem to be in a dour mood today.]