A) Presumably he is trespassing, so the law is on your side and I would attempt to catch him in the act and ask him why he is trying to sabotage your efforts. Hopefully, his embarrassment and reluctance to be caught again and perhaps face a more robust confrontation, may stop him. Film and record any confrontation, but do not in any way threaten him. I would take further legal advice and hope that the matter can be settled amicably.