I have read several articles asking us to write down the name of the one whom you hate and just scribble over it. This way you could quench the hatred towards them. Have tried this out? I have! The more I did this the more hatred I developed and the more negative my thoughts became. It might work for someone but for me it was the worst idea I could ever implement in my life. Tearing the names or punching them or poking them evokes a strange negativity in my mind which was detrimental to my sanity.
If you are one who have tried and failed in this technique, you are my personality type. The best way to get rid of hatred towards someone is to stop thinking of them. It would not be possible at all times. Sometimes, this someone would be in front of you all the time and hence stop thinking would not be a good idea. Stop letting these thoughts affect you is what you could try doing.
This is not a instant job. To not let such thoughts affect you will take time depending on the influence this person has on your life and psyche. Hatred is one such emotion that once get connected with you, will never budge. The first step to take is to think of this person as a fourth person about whom you have heard. This is more of a psychological exercise. The scientific authenticity of the same is not verified. I am sharing what helped me and could help you as well. We detach ourselves from them and start thinking of them as a stranger about whom you have heard things which are derogatory. This is a gradual yet continuous exercise. Once we are into the groove of this exercise, at a point you will forget the exact reason why you hate them. Well, not in the literal sense though. And then if you look back, you will see for yourself how immature it was to reserve a major portion of you brain for something that doesn’t matter to you at all.