Unfortunately there are a small percentage of people who are flat out nutcases. Best policy is to avoid any interaction whatsoever with them, pass and gone. If for some reason you do have to interact with them, I've found that it's sometimes to rely on pro forma social rituals, triggering the conditioned responses which they hopefully learned in childhood.

I can't always tell while approaching from behind that it may be a socially deranged individual, barring obvious erratic behavior. That's why I like the bell, taking a page out of Pavlov's book. Ring the bell, then pass politely with some form of formal social acknowledgement. I think that the formality is critical because the response is already learned and habitual, while avoiding familiarity. It conditions him to associate the sound with a successful social interaction, and soon only the bell is needed to induce the same positive emotional response.