"Brand terrorist" is a phrase you sometimes hear in the customer experience world. It describes a customer who dislikes a company and actively spreads bad word-of-mouth.
I've always felt this metaphor is way over-the-top. "Terrorist" is far too extreme a word to describe someone who merely says mean things about you. Maybe others feel the same, which would epxlain why it hasn't really caught on.
On the other hand....When someone smashes a $400,000 sports car with a sledgehammer in public in order to complain about bad service, "brand terrorist" starts to feel about right.