Justification for vegetarian

I'm sure its been discussed before.. but how does everyone reply to the people on your page that comment and tell you that your being vegan doesn't change things? Simply because somewhere during the process of our food its still destroying habitats and stuff.. or that the farms used to produce our food is ran by animals pulling and plowing?

 This happens every time I post anything to my page about veganism and its seriously annoying me.. Try an analogy, although I find that many non vegans do not understand analogy. For instance, you can say: "Just because I happen to unknowingly buy a shirt that may have been made by slave labor is not justification for me to kill my neighbor because I want his car."