Very rude customer spreading rants on the internet?

We had this potential customer called us. He was asking about our service. The tone is very rude and impatient. At one point, he was asking a very specific question about how we do the service. We cannot explain in detail how we do our service especially to a customer we have not met. So we said "sorry we cannot explain it over the phone sir". Suddenly he mumbled and hang out the phone. Later we found out, this male customer is spreading rants and negative comments about our company on the internet. We thought he has not used our service, how can he said about the negative thing. It's just pure childish act. And we don't like this guy spreading negative comments on the internet. what can we do?

A slander Law suit will shut his loud mouth.

I don't know what he's saying, but if he's just talking about how he doesn't like your company, it's not libel/slander and you can't sue him. Everyone is entitled to express an opinion, even an unfounded one. There's not much you can do.

Probably nothing you can do about it unless you have the links to prove all of his slandering. It could be illegal, but again not sure. To tell you the truth I get pretty pissed off sometimes if I've had to wait forever to speak to a human being over the phone and when I finally get one, they say they can't help me or the response is, "I don't know" or "we can't explain it over the phone , you'll have to come in to the store." Maybe they don't know the person who called may live 50 miles away and can't run to the store now. Maybe they wanted to find out something so that it saves them a 50 mile trip, just for example. It's frustrating.

A good PR move would be for your company to put a good FAQ/explanation of the service, or at least why you can't do estimates over the phone, on your website and answer his ranting by saying "We're sorry that you feel this way, but you can find more information at our website here..."

This move tells people that you are not only confident in your service, but that you strive to satisfy all customers, even the jerks.

The threat of a libel suit (for written harassment as opposed to spoken) might cool his jets.

internet is faceless. and when people rant and spread negative or critical comments, they are always dismissed. there is no real value to them, as they are ranters.
this person can complain away and no one will care.
it will stop soon.

hire nicholas cage to go bangkok dangerous on his ass

Sue him.