There was once an idea that was taught to us as Younglings. It transcended nationality, race, gender, and religion. That concept was, "If you can't say something nice, don't say anything at all'.
Seems like a great many people either have forgotten this, or never learned it. Now, of course I see the value in constructive criticism, but this goes FAR beyond anything that can be used for a positive.
When people poo-poo something, just to bitch and complain, how do you usually handle it?
Depends upon the context, the level of hostility and whether or not I have to encounter said unpleasant person on a regular basis. I try not to escalate.
Of course, there's always the tried and true: I'm rubber, you're glue - it bounces off of me and sticks to you.