What's Society to do?

So I moved in with my boyfriend who lives right in Davis Sq. slightly closer to Powderhouse circle (I used to be near Porter). I've lived in Somerville for close to 4 years.

So now for the dilemma. There is a man who I believe is named Mike who is clearly mentally disabled. Some days he sits quietly on a bench and chain smokes, but more often than not he follows people down the street yelling, screaming, spitting, swearing at them and violating their personal space while doing it. Most of the time he repeats the same speech - why did they do this or that - not really sure what he's saying and I really try to avoid him at all costs.

Now I really wouldn't care if this man sat on a bench all day and yelled and smoked. But he doesn't he walks down the streets scaring people (at least it scares me) - he talks way too closely for my comfort and spits on you as he's yelling (not on purpose). But it's gross - who wants to get spit on?

Now he's coming further down my street towards Davis (he used to stay right @Powerhouse circle). Now most mornings I wake up to him screaming outside my window. It's annoying when my windows are open in the summer and all I hear for about 2 hours straight are the same ramblings from this guy. I would like to wake up to a little peace and quiet - my street is busy enough with all the cars.

And now last week he started a new thing - screaming 'get the fuck out of here' over and over. Great... now he's yelling AND angry AND swearing! AND on MY fucking front lawn! I don't want to be harassed and scared when I'm trying to come and go from my home! I don't want to feel like a hostage in my own home because I'm afraid to leave because he's standing in front of my car. I don't want someone screaming get the fuck out of here at me while I am trying to get into my car to go to work! This morning he was hovering on the sidewalk in front of my house - blocking the way to my car so I stayed in my house until he moved across the street to yell at another innocent victim.

I mean what is one to do about this? On one hand the man is clearly mentally disabled. He clearly can't help it - I feel bad for him. But then why is it ok for him (or anyone) to scream at me in front of my home? BF says to me well he's harmless he's never hurt anyone, just ignore him. It seems he hasn't, but still that makes it ok for him to verbally assault me in front of my home because he's got an illness? Where are my rights? How can I ignore someone yelling at me, close to my face, spitting on me, walking next to me? How does society handle something like this? What is the right thing to do here?