Brain Bleach/Oral Tradition

In the Greek tragedy Oedipus the King, when Oedipus learned that he had murdered the King on the road before marrying the Queen, oh, and also that they're his mom and dad, which means his kids are the product of incest, he literally gouged out his own eyes in horror on finding her dead by her own hand (using the pin used to secure his cloak, no less), and then went Walking the Earth to atone for what he had done, making this Older Than Feudalism.
There's also nepenthe, which really is Brain Bleach.
This is the express purpose of the waters of the river Lethe—to utterly remove one's memories of his or her past life.
At the siege of Thebes, Athena is just about to bring Tydeus immortality, when she finds him chewing on the brains of the guy he just killed. Before she can go back home, she has to scrub her brain out with a ceremonial fire of purification and water from a special river. Needless to say, no immortality for you, Tydeus.