You're asking for a magical resolution. Do not act using any of mantra before you prove you are a man. Stop searching solution in mantra because despite your good intention you will attract more bad karma on you. Be a man and prove he is a liar. If necessary use the same way on him: speak with others about his small dick, his dumb head and so and so... The true: he is not your friend. Stop giving him attention and focus on your love relation. I am sure this way will made him stop sooner than you expect. Now, give away my previous jokes. It seems you're at an earlier age. As young, we made mistakes. Be sure the girl is really involved with you. Because if your relation is strong, nothing can stop it to evolve. Are you really in love with her, or you don't really interact with her? If you have a love affair, a sexual relation I can not figure how a little "preach" like other guy can obstruct you. The idea he will go far away from the town, and so, are childish and puerile things. So go deeper in you relation, be mature and act like a man. I can give you the mantra to harm this guy, but, really, this is not the solution. Resolve the social conflict before attending any practice. The fight is not a solution. Maybe the "words" spreading around will return to him and put him in a bad light. Maybe the use of force is not a solution, but sometimes you need "stronger" friends around you.