When it is people on the other team doing trash talking, I don't care. That can be kind of fun, actually.
When it is someone on my own team who thinks saying something mean to me is going to help win the game, I respond by throwing the game. If they thought I was bad before, just wait until they see me on Zenyatta rushing the front lines over and over while missing all my attacks. I figure by playing as bad as possible maybe I can make them even angrier, and that counts as a win in my books.
So yeah, I respond to toxicity by being even more toxic back. Not with words, though. I don't like to say mean things to people. I will be toxic with actions instead.