Maybe stop using a positive.
Use a negative. It seems like that may be the only thing that gets through. When you reiterate when you ARE there, that doesn't tell him to stop.
Say, "No, I will not be here after hours. I'm sorry I can't help you. If you don't want to come in early (since I'm here before you), then you'll need to find someone else. Perhaps you can try a real-estate office or the Yellow Pages. Good luck."
And then never respond when he asks repeatedly.