The problem with spikes is that if you have livestock or allow horses to cross your property then you may injure them. You may also injure wildlife.
I don't think wire at neck level would constitute murder. Manslaughter at most. It would be hard to prove that the wire was intended to kill rather than deter.
I think signs might be a deterrent for honest folk. If you are worried about dishonest folk, then something along the lines of 'private hunt club. do not enter. stray bullet hazard.' and then shooting off a few large-caliber blanks whenever you hear ATV noise, might be a workable trick.
You might also find out if there is ATV 'habitat' that these people are migrating to from other ATV 'habitat'. If your land is the quickest route between two BLM ATV sites or private ATV parks, then your best bet might be to make a designated ATV path from site A to B with appropriate warnings so you aren't liable if they hurt themselves, and a prohibition from leaving the designated path.
This behavior is mind-boggling to me, because when I was a kid we used to ride horses across other people's land only when we had permission or when the route was a generally-accepted route not posted against tresspassing. There was, for example, a defunct county road along the bottom of someone's property along a creekbed and old quarry, that connected a horsey neighborhood with a livery stable that had about 500 acres of riding trails backing onto a county park. It just wasn't thinkable to deviate off that trail into the guys property, even though it wasn't posted against trespassing. All the horse community there knew that the guy was afraid of liability and so everyone respected his wishes.
Talking to the kid was good because that might help get the word out that the ATV's are unwelcome (as if you didn't know that...
) . A letter to the local paper also might help bring attention to the issue, especially if the newspaper can do a feature on where ATV's are allowed so that new riders know where to go.
