A metatroll is someone who is trolling someone else who is clearly a troll.
Person 1 (troll)- Man, I hate prank sites so much. That last one I saw was gross. They're just so stupid. They take no thought or consideration to make; everyone who likes them is an idiot.
Person 2(metatroll)- Hey, you should try going to this site called pen island. It's got a lot of people who think the same way as you.
Person 3- Dude, you're trolling a troll right now. You're totally metatrolling.