2x4's with nails driven through them are great. Just bury them a little under the sand/dirt. you can even write a nice saying to the violators on the back side of the board. Just place them strategically so that the oil well pumpers don't get into them.
This was my suggestion to my brother, but it'll only work on the cut through he made. If I put anything on the gravel road it'll get the oil guys first.