Old Man with His Dog + Shit

There is this old man around my in-law's house, who likes to bring his dog to shit and pee at neighbours' house. He is really irritating.

My MIL saw him doing it again the other day, ON our staircase, she asked him to go elsewhere. Guess what he said?

Old man: "Pee only mah, why are you so concrned about it?" (小便而已, 这么紧张做什么?)

MIL: "Wah, pee ONLY? Since you are ok with it why don't you let your dog do it in your house? What about letting my dog pee at your house? Are you ok with that?"

This old man then walked over to the other neighbour and let his dog pee and shit there. MIL also asked him not to do, but he just ignored.

And then... this fella, he keeps coming back to our house, trying to challenge us. Whenever MIL can't see him, we will have the shit/pee. I intend to snap his photo, catching him doing this in action. Not sure our law is the same as HK? The other day Dodo Cheng hosted a program said we can sue this dog owner... Wonder if I can use his photo to sue him and teach him a lesson?! Terrible old man.