Spread the rumor that they had sex with *insert female name here*. It wont work if the girl you pick is a slut, and nobody will believe you if you pick an ugly biotch.
But what I did, was this kid that used to be my close friend, one day, when he started talking to me, I would just yell at him "Shut the fuck up Tony! Your a dumbass!". And whenever he tried to talk to me, I would smart something like that at him. He wasn't particulary popular, but nobody hated him. After a few months of just shoving him away, physically, whenever he came to speak to me, or telling him he was a jerk, everybody grew to hate him. It's been like that ever since, nobody likes the kid, all the girls tell him that he's an ugly little fuckhole, so on so forth. I did this because he was starting to be a real pest, and hummiliated me by spreading pictures of a newspaper incident featuring me, not to be spoken. I didn't mean for everyone to hate him, but that's the way things turned out. I'm trying to be nice to him now, since I didn't want everyone to hate him, but the problem is all my friends hate him.
Oh well, that really was a great way to ruin somebodies life. Great as in it worked like a charm.