Pen them up at night if you can. Also another idea from my own experience. I had some trouble like this years ago and suspected someone was stealing ducks. I put up some "security cameras in use" signs and bought one of those cheep fake security cameras that has a red blinking light on it and put it up. Must have convinced them because the disappearing ducks stopped. Also "No Trespassing" or "Trespassers will be prosecuted" signs may help.