Do they do it at regular times?  Take up skeet shooting, hunt for partridges, shoot crows, or whatever at the right times.  We've cured quite a few trespassing problems by being present with shotguns at convenient times.  Make sure you're pleasant when you're talking with everybody you meet.  If folks know you're around and armed, they're much more likely to respect your property.  There's no need to get violent or unpleasant.  Folks think twice or three times about trespassing when you are "polite".  Make sure you give them a wide grin and a knowing look.  If someone asks what the shotgun is for, tell 'em crop protection.  Around here we're allowed to carry to shoot varmints that damage the crop.

 p.s. Skip the wires.  I had someone I knew beheaded by a cable strung across a road by a landowner.  The cable wasn't marked, either.  The only thing that saved the landowner was the fact that he had a "No Trespassing" sign drying in his garage.