I think it's more fear and ignorance than predudice and I would have maybe tried to educate the people rather than just stop the conversation; people are going to think what they're going to think and getting it out in the "open" is the only way it can be dealt with and "helped."

I would have pointed out the pain the people are in, how they're ostracized by people like you (the ignorant people commenting) because of their fear and ignorance and only then would I point out that I was like them and that "I get along with you all right, don't I?" and tried to get them to see more sides to the equation, that the person self-harming was not some monster wholly unlike them; that being handicapped can happen at any time (I have a good friend who is quadriplegic because he decided to dive off a bridge into the river as he had many times before when he was 19 only the tide was out) and "how would you like it if I made fun of you after you were in a car accident or had a stroke?"