A recent "Ask Marilyn" column featured a question about whether or not a person who had traumatic or painful memories could be hypnotized to forget those memories. vos Savant (nee Mach) wrote that this would not be possible because "memories are chemical, meaning they have substance."
She said a person who was susceptible to hypnotism could "... respond to the suggestion to 'forget' certain events... but this action simply prevents them from being able to recall all the episodes. The memory itself still exists in their brains, and so does the aftermath and the many relevant associations... The result is that the people still feel bad but cannot recall why."