Don't do it.
No one likes bad reviews. They sting. But the thing about the Internet is that it gives everyone a voice. Which is a good thing. Hell, it's a great thing. It means that people who may have no other outlet will have a way to reach the masses.
It also means that every single person who has an opinion on something has a way to shout it to the world. Maybe you don't like that opinion. Maybe you think it's unfair or unjustified. Maybe you're even right.
But you should never, ever, EVER, respond directly to a bad review. Never call a person or a website or a reviewer out by name. It doesn't matter what your intentions are or what justification you use, you will always look bad.
If you read a bad review and you feel like you HAVE to respond, then write out that response. Write every horrible thing you want to say. And then send it to your mother. But don't put it up for the world to see. And don't make comments anonymously; everyone will know it's you.
Just step away from the keyboard and let it go. Your career will thank you.