First rule of ANY review - good or bad - is to reply PROFESSIONALLY, HONESTLY and COURTEOUSLY. There are 4 reasons for this.

First, you encourage those who have left positive reviews by thanking them and adding a little something about why you enjoyed their project.

Second, by adding something about their project, you showcase you areas of expertise and you'll likely jog someone's memory who will say, "Oh yeah, that's exactly what I need!"

Third, for bad reviews, it gives you a chance for damage control. But remember, honesty. If you screwed up, own up! Say what you did to remedy it. If it's false explain it. An honest, polite reply to a bad review will still show you in a good light to future prospects.

Fourth, Google loves seeing replies. Why? Because it tells them (right or wrong) that you're an engaged and active business person who cares about their online reputation. Online reviews are great but so are replies to them.

After that, unless the material is libelous there's not much you can do. As the Brits say, "Keep calm and carry on!"