You would have to research the laws in your state.
Keeping people off a privately owned section of land can be a real problem, when the owner wants people to stay out. There is always this sense of entitlement by those who trespass.
Boards with nails, the nails bend over, sure it can work, but something more substantial will work much better.
As I understand the law here in NYS, and you have to realize its ones interpretation to an extent, if the land is conspicuously posted, the owner has the right to expect people will stay out, so if someone does drive a vehicle in and gets a flat tire, I don't see how an owner is in any way responsible. That's the basic premise, vehicle and traffic law applies with ATV's 2403 - forget the subsection, as well as penal law.
The above regarding the signage being posted conspicuously. to me, means that if I place real tire spikes out there, and someone who is clearly trespassing, their vehicle is disabled, I am responsible, prove it ! I have the expectation the signs will be obeyed, well that's what the law says.
In my situation, I try to handle these problems without having to resort to aggressive means, and I don't really have much trouble, but I have employed the above devices and have seen the results, when theft occurs, signs were vandalized, stolen and fences were destroyed.
If people are so determined to violate the law, then they will be met with further determination to stop them in their tracks, its just how it is.
Liability, your flat tires are my problem and you broke the law, I don't get that, you have no right to enter said property, its clearly posted, if I am nice I may add a warning that certain devices are employed in undisclosed places, if you cross this line, its at your own risk.
Now if you stretch a thin cable across a known path, or otherwise employ booby traps that kill or maim, that's another ball game, I don't see that being very wise for any reason at all.
I find its best to see what is going on, and take the appropriate measures, most times its a simple conversation, other times you may want to be extremely careful, not be seen, observe, I've seen locals overrun a friends place, and it got real ugly, very dangerous and a very uneasy place to be.