From the point raised by leg11aug and Hugh, I would determine that there is (with all due respect) a lack of understanding of the causes of the problem.
If an area is being 'hit' by nighthawks (s**thawks would have been a much more appropriate name) then there is a high probability that there are metal artefacts being found there. Whether the site has burials or not, is scheduled or not, is of no consequence to these people. All they want are items they can sell. They don't care about the archaeology, or the ethics, or anything else (including the Law or the farmer) other than the 'bling' factor.
Farmers, in general, are aware of these people, and are more annoyed at the damage they cause to the crops than what is taken out of the ground. The incidents sometimes get reported, and county archaeologists informed, especially if scheduled sites are affected. But, as with everything else involving Heritage, there is no way of enforcing any restrictions on these people. Consider this... What would happen if a farmer and county archaeologist actually caught some of these people in the act? What law or punishment would be used in such a case? There is no deterent that I know of to prevent these people doing what they do. They are not considered 'Detectorists' by us, in much the same way as grave robbers are considered 'Archaeologists'.
So what solutions do we have available? Leg11aug has named the only 3 that I can think of. First may be financially restrictive, and the third impractical because of the 'threat'. Which leaves the second option. Deny the thieves any prospect of loot, and they won't bother with the site. It won't be worth their time.
Hosty mentioned a working depth of 10". This is true for most machines. But some of you may be shocked to know that a couple of the newer Detectors can reach depths of 15" and there is rumour of a new detector that can detect metal to a depth of a metre. I say rumour because it boils down to sales pitch. GaryBrun or Steve-B could give you more information on these machines.
If a complete excavation is financially restrictive, would the removal of all metal, which could be recorded using a GPS by 'qualified' people be of practical use? After all, it is the removal of these items from the site, without adequate recording, that has caused most of the arguments between detectorists and archaeologists.
Mercenary. An interesting point, but the reason the 'scrap' is removed from the area is at the request of the farmer. I believe (but don't know for sure) the corroding metal effects the pH of the soil, so the farmers are happy to see it removed. Sometimes the items are kept by the farmer because they came from one of his tractors...
. Leg11aug is spot on about detectorists revisiting 'productive' sites. But the finding of reburied scrap metal is seen more in the light of bad-manners or laziness on the part of the original finder and is subsequently removed.
I hope I've answered some of the points raised to the satisfaction of the questioner, and that the way in which we conduct searches makes more sense to some people.