Once you choose to become cold hearted toward someone, you must remain firm in your decision to do so. Keep reminding yourself why it is critical to your own well-being and happiness that you cut ties and close off your affections for this person.  Be crystal clear when you make the break: “I’ve realized that this relationship is harmful to me, and I have decided that we cannot be together any longer. This is non-negotiable.” Cutting ties in this fashion is a harsh and challenging measure, so save it for instances in which a relationship is clearly detrimental and irreparable. If possible, do not accept phone calls, emails, Facebook messages, etc. Every time you give a chance for them to explain or apologize, your resolve may be weakened.  Explain the change once and only once: “I don’t want us to have any contact unless it is absolutely necessary.” You must view each and every attempt at reconciliation as the other person’s desire to put you back into a position to hurt you again. When forced to communicate with the person, use “no” as a complete sentence if they ask if they can call you, if they can explain things, and so on. Otherwise, give short, direct replies such as “I can’t,” “That isn’t possible,” or “I don't have time for that.” Then walk away, hang up, etc.  Or, pretend not to notice or hear them at all. Use these tactics when dealing with a co-worker or classmate, for instance. Do not express anything that could be construed as doubt or remorse, such as saying “sorry” whether you mean it or not. Show no signs of lingering affection. Don’t even glance their way. Close yourself off completely.  Don’t get dragged into discussions about how you’re going to move on, what you’re planning to do next, and so forth. Become a complete mystery to them. You've already expressed your decision to them. You don't owe them anything more than that, no matter what they say. Do not reflect on the "good times" you might have shared. You can’t be cold hearted while having fond memories.  Delete all texts, emails, etc....

TL;DR in Indonesian:
Ambillah keputusan lalu lakukan. Jangan berinteraksi dengannya. Lakukan percakapan sesingkat mungkin. Jangan biarkan ia tahu apa yang sedang Anda pikirkan atau rasakan. Jangan bernostalgia atau terbawa perasaan.