There really isn't much the LL or police can do sadly. This is bullying to the extreme and it is sad that this is happening for such a prolonged stretch. You are not to blame for this. They are displaying extreme psychopathic behaviour and have found a target they can lash out at.

I'm assuming you have visited this site.

It is important not to confront. Let the cat be. Keep your door locked and stay in your room to avoid confrontation. Don't use the common areas unless necessary. Try to avoid expressing emotion. No matter what they try, turn away and leave. Remember, nothing anyone SAYS to you can hurt you. It is their actions that can, they are not there and it appears at face value they are not going to go there. Those aren't really threats, they are intimidation.

As said, you should visit or talk to the police. They are trained in how to deal with situations like this.