What kind of language is that, Finnish? Anyway, I'd ignore if I were you, but if I knew this guy IRL I'd be careful when turning my back at him, and I'd avoid falling asleep when he's around.
Maybe its better just to kill him and be done with it. Call it proactive self defense.
It's like never going to the toilet when Tyrion is around - avoid encounters with him at all cost. I'm not sure if your "proactive self defense" would hold in court though. Why bother risking getting a criminal record and going to jail because of a person like that?
Best way is to identify him, and spread it on social media so that everyone would be warned.