I don't fight with them, but i have quietly unfriended and blocked people who act that way. Usually, it's not just because they bash my little car. I can just not approve those comments. It's more that they are just useless people who never post anything I want to read anyway.
The same people who call someone a douche for driving a hummer (because it's a giant 8000 pound incapable pile of crap that gets 14 mpg), don't seem to scale their political views past vehicles the size of a Civic. Somehow, that's the range of their environmental correctness. In the end, with these people, I believe that it comes down to simple jealousy. They see the cars at both ends of the spectrum as toys that they do not have. It makes them feel inadequate, and they lash out. I know that before I owned a smart, and I saw someone driving one, I always assumed they were well off, and that they also had a "normal" car. I didn't see the utility in a smart at all, and never even considered that one could be a full time, 12000 mile a year, daily driver. I don't think that I'd have ever bought one had I not also had a truck in the driveway. Now, though, a year and 12000 miles later, that truck is just in my way most of the time. It is driven so infrequently, that my HOA thought it was inoperable, and tried to fine me for it.
People who don't own a smart, don't see them this way, though, and the jealousy leads to all sorts of infantile behavior.