if you says to him that you have aids (or somthing like that) ha would have to tell hiss best friend( i asume that his frien had sex with the younger girl) and then all of them have to take tasts and shit.. or tell his perents....
i like the herpes idea. or tell him u got aids (just a little scarrier than herpes). destroy his car, have him catch u cheating on him with another girl (he'll be gone 4 sure), light his lawn on fire always fun, i dont know wat its called, but its a drug that flushes out ur system (u puke and shit like crazy) throw some in his food and then write "FUCK U" on the toliet paper
this drug is called epicac, easy to get, people have it so if a baby ate somthin toxic, they give it to them so they can throw it up, mix some of this into one of his drinks, hel be out for a while