I slowly lost all my zoas and Firefishes for over an year, until I spotted its cave entrance and started monitoring the tank at night. Left a piece of shrimp near the cave and voilá, the bobbit worm came out.
Then the battle began. I tried to capture it with a trap, no success, like the others here.
My solution was to remove to rock and leave it in fresh water for a couple of days, until it came out dead. Interesting enough, alive it was around 30cm, dead it was a little ball less than 1cm long.