"Trolling" is generally defined as intentionally saying ignorant or rude things online to incite anger in others for one's own amusement. However, Reverse Trolling is just as fun and perhaps more successful.
It boils down to several types of Reverse Trolling. Overall, Reverse Trolling involves accusing someone else of being a troll in an attempt to invalidate their opinions.
This is done in several ways. The first is simple. Someone posts an opinion. Let's say they suggest that people who eat beef are more responsible for global warming because of the methane from cows. Your reply is: "You're just trolling.... right?" This puts them on the defensive. You're suggesting that their opinions are so stupid, so poorly thought out, that it is impossible for ANYONE to actually think what they do. The troll has to either insist they are not a troll, or to admit they are troll. In the eyes of the Reverse Troll, those are the only two responses the troll may give to them.
The second style of reverse trolling is more difficult, because it's less likely they are actually trolling. Simply, if someone says something that personally offends or upsets you and challenges your worldview, you write them off as a troll so you don't have to listen to anything they said. This style of reverse trolling is more defensive than offensive, and less effective in the cyber battlefield.
The third style of reverse trolling is the the most effective at defeating a troll, but it takes a long amount of time, and thus will only be used by a select few. Instead of immediately calling someone with idiotic opinions of troll, you engage in a probably fruitless attempt to educate them about (the theory of evolution/basic economics/goat herding) knowing full well that they are probably a troll. Doing this "education" while being abnormally polite can diffuse a troll.
They were hoping to enrage someone with their opinion, and instead they're trapped in a long discussion with someone that isn't providing them with what they want. If you are patient enough, even the most dedicated troll will give up if you keep rambling at them.
Reverse trolling is an undervalued aspect of internet communication, and I encourage it.