Paint the person a monster. Everybody hates a monster, even worthless good for nothing bums love to heckle them. This is a game that has been played by crooks again and again, rival in love? Start whispering!
“I saw that person in the park with a girl of dubious reputation… you know… yeah that type of woman!” then if challenged, “I thought I saw that guy… maybe I was wrong! You see …” but if not challenged then he gets the deal and the rumor spreads till it reaches their love-interest’s ear in a twisted form.
Or just say something about his brother, mother, father… things you have heard from others… now rumors always have some basis, don’t they? That is what all rumor mongers say! You have just heard it, you don’t know how authentic it is.
You just say all sorts of unsavory things and then let others carry it forth till it reaches the proportion of disgusting and reaches the desired shores. If it works then you win, if it does not well, at least the rumors will stay and their lives together won’t be that peachy! As perfect and beautiful as they were supposed to be!