Now,as you give more details, the problem looks a serious one. Anyway, how this guy knows your fiance new numbers? This is one problem. They had common friends, maybe? Or how. I don't know how is life in India, the true is you're far away from western point of views and I will not give you advices in a social system I am not involved by direct experience. But you must separate your life and suppress any contact with this guy, you and your girlfriend too. If you both ignore him, he will stop. If you give attention to a bad people, you just nourish his bad intentions. It is clear for me the fact this guy is a trash. Throw the trash out from your life. Treat him like the last man on the earth, don't pay attention. I don't know how a girl past has influence in daily life in India. Peoples talking always bad things about others. If you really want to stop those things, and you want to practice mantra yoga for this purpose, here's the link to Bagalamukhi Yantra and Mantra sadhana. But keep in mind, you do this thing to stop for a while others actions, but if there is a social problem, it will never end till you act in this social world. I could tell you: Take this girlfriend as wife. But I don't know and I don't want to enter in others life. Keep in mind you practice in secret (don't talk with anyone, I mean even with your girlfriend) about your meditation. And keep in mind you must practice this with the will of being protected and not harming others. Here is the link
http://mantra.ro-design.net/bagalamu...ra-mantra.html
If you have any question, leave a private message and I will answer if I can help you. Use the second yantra given, if you want a bigger image to print out I will provide you a link.
A few words for practice. No one must see the yantra you use. So keep it private. The first step in practice is to calm down your mind, at a level when words said by this guy and even your fiance will stop coming in mind (yes they will come because are impressions or samskaras). The second stage is to avoid calling in mind this guy and your fiance images. So at a level of practice, you will feel complete out of any relation. Then after those are done, you can visualize other images. But those things I will tell you in private. For other peoples reading this post, keep your comments for you and don't bother to propose money exchange for offering a solution. No need for extra "guru" here or "money affair". Because I believe the scope of any practice is union with God. He not asks for money. So the spiritual practice must be free of "world" affairs.