I don't think there really is any spell to make you forget someone.
Sometimes, your body has the ability to forget traumatic experiences because the mind considers the pain that follows the memory as a threat to the body, ergo, the mind shuts out that memory, alowing you to forget.
The best thing to do is to reprocess what happened until the pain of the memory goes away naturally. If the memory is too greatly painful to reprocess, or if you're not sure how to do it correctly, there are other methods you can use as an alternative.
It's not a very good idea to use amnesia to forget something.
Reason why is because there are some key things you can learn from the memory of the person. Learn from your mistakes as we call it. If you use amnesia to forget, you could most likely make the same mistake again and suffer greatly a second time.