Trolls are people of arguable intelligence and possessors of an abundance of free time that lurk around the MAL forum searching for fun. There are many types of trolls in MAL, but the user must be aware that trolls are not the same as trolls wanna-be . Those are usually annoying users that have no ability with trolling whatsoever. Some notable trolls are Okazaki , Lupadim and DramaEnthusiast.
Undoubtedly, the most infamous of these"trolls" would be TrueKireiPriest and his merry band of Yuetsu members. These include ZeroDragon, CarenPriest, BloodRequiem, and Shrimperor. All others have been deemed second-rate, and lie in a state of limbo where they are either discovered by TrueKireiPriest or left to be smote by Lord Nakata's wrath.
It is really difficult to identify a good troll. You can't know if he is actually serious or not. Many mods have this difficulty, so some of them can't act. A few mods, known as the sniper mods, just ban every single user that seems to be a troll.
There is no way to counter a troll. The best you can do is ignore them.