If you must engage a troll (and I usually advise against it), the best thing you can do is completely ignore whatever it is the troll rambling about, hijack the thread, and turn it into a conversation about the hit '70s television show Three's Company.
Troll: "STFU loser, ur teh dumb! [Insert troll's unsolicited extreme view on abortion/gay marriage/gun control/the president that has nothing to do with the initial post]."
Response: "Even with the addition of veteran actor Don Knotts to the cast, I feel like Three's Company really jumped the shark after the Ropers were spun-off into their own sitcom. The show really peaked during the Suzanne Somers years!"
Nine times out of ten, the results are highly amusing.