Even though you think you can control how the memories are formed while playing the role of an observer, you don't really have control on certain kind of memories, for example, memories which have formed while you and I were being born in our mother's womb. These kinds of memories make us who we are and not only our brains have these memories but also each cell in our body have these memories, and it is passed on through generations.

So, when talking about memories made by observing something can be deleted/forgotten by simply ignoring them. And, our brains help us in making the neuronic connection weak to those memories and in result helping us deleting them from our minds.