Wow, I am sorry to hear that. You should really just be the better man, and walk away. You can do much better, and its not worth your time getting revenge. Spend that time doing other stuff, like finding another girl OR getting bigger in the gym. However if you insist on making some girls life horrible then you coudl do the following:
1) Go to Barnes& Nobles, and grab all those free magazine postcards out of all the mags they have. Dont worry they are free to take. Then mail them for a years subscription.
2) Deflate all her TIRES in her car. Thats a bizatch to fix.
3) I have heard people do some pretty nasty stuff to unlocked cars. (Use your imaginization)
4) Date a more attractive girl!
5) Walk away and show her you dont care :-D