I respond in some neutral way, not sounding angry at all, typically being honest when addressing their comment, and it's hilarious how quickly they turn around. One second, they're calling me out, I respond calmly and seriously, and then they name themselves out to being the jerk and often apologize.
Though I do have to admit, I did have this nice homophobic troll on one of my videos and I did have some fun with that guy. That one was too much to pass up. Unfortunately they deleted some of their comments, so the convo thread isn't as great as it was.