Of course, if you really wanted to hurt her... Say you like one of her friends, one that doesn't mind you/being around you/might go out with you (even if you DON'T like the new girl) despite what she says. This will make her fairly mad. If it's not enough, then tell her about the fun you and your new friend had on any given day, but without making it seem like you're talking just to piss her off. Keep this up, getting closer and closer with your new girlfriend, and watch your ex's anger unfold. Do this repeatedly until you're either tired of going out/being friends with your new girl, or you think your ex has suffered enough, or your new girl has come to her senses in realising that she's being used and dumps you, then go to your ex and tell her you did it just to get back at her. She'll be really mad... But if you ever want a relationship with a chick that knows her ever again, you might wanna start reeling the newest chick in before you tell your ex, just talk to her a tiny bit, show her you can be trusted, etc etc, otherwise, your ex will possibly ruin your name and you'll have to find someone else, who doesn't know the first ex at all...