What an excellent quiestion! Of course one cannot erase something terrible from the mind unless it was so traumatic that he doesn’t realized it ever happened. However, that same person could try filling his mind and free time with worthy pursuits , so he has little time to think of his aveira, except potentially if ever confronted with that yetzer hara again (and one would hope that his memory would protect him from being nichsahl again).
It is sad, but one of the punishments for committing devastating aveiras, is often the guilt that accompanies them . Guilt is a powerful thing. A truly remorseful person cannot just erase it, but perhaps at least hope to learn from his former actions and become a better person for it. hashem forgives, but we are not Hashem and it is not always easy to forgive ourselves. We have to just go on and hopefully walk a more Torahdik path, and that should give the person some nechama.