Wow. I just read the letter. I don't see anything illegal in it that I know of (Canada I have no idea). I actually got one like it from a bully as a child. It shook me up so much that I showed my parents and they were dumfounded at the cruelty (I'm not autistic, but it had similar, "you should just die and no one will ever like you" type stuff). There was no recourse of course. It was just a cruel letter. They didn't opt to try to get the child expelled or anything, it was just a lesson in life that some people are extremely cruel.
This letter is obviously written by a real-hole who will be VERY EMBARRASSED once his/her identity is divulged. If/when that happens, they will be the ones moving because the neighborhood will know how incredibly awful the person is. Even if other community members are annoyed by the child, they would not condone such cruelty. Unless the neighborhood is some bizarre alternative universe of cruel people.
To me it looks like a case of a really mean letter. Can lawyers spin it into a legal case? There may be a way if it damages someone....but I don't think so in my humble opinion. If I had received a letter like this, I would have copied it an placed it on every door in the neighborhood (as well as putting it online) asking who had written it so the whole "anonymous" angle was blown. I'm sure the person who wrote it complains all the time to other neighbors and everyone knows who did it. They'll get their punishment even if it's not by any authorities.
Also, the sentiments here are downright Third Reich-ish. The author basically insults every disabled person on the planet. So that person will have to just move and change their name pretty much.