Every pirate that pirates your game and likes it pays for it with free advertising, spreading the word to other gamers, some of whom will then buy the game.
Piracy is a hugely overrated concern and more often than not it actually backfires, causing grief and frustration to legit gamers. Latest example of that -- Sim City.
Bottom line is: worry about making a great game first, and only once it becomes popular should you think about something to separate your legit gamers from pirates. Never harm pirates. Reward legit gamers instead with bonuses that pirates can't get.