Would an ablative neurosurgery that causes amnesia be both possible and ethical?

Ablative brain: surgery does have a role in modern, effective medical practice. This technique, where targeted brain tissues is destroyed by various methods, is most often used in treating Parkinson's Disease; also used in cluster headache. Amnesia can result. It has been used for psychiatric disorders, but is currently rarely done in the United States; there are significant ethical issues as you suggest.

No and no.: I wouldn't think that theoretically it would be possible with any degree of accuracy, and in any event it would certainly not be ethical to impose amnesia upon someone surgically.

Patient H.M.: H.M., or Henry Molaison, is one of the most famous cases in medical history. He had both of his mesial temporal lobes removed in an attempt to cure his epilepsy. Unfortunately, it caused a profound amnesia. Read about him and you'll understand why no surgeon will ever remove both mesial temporal lobes from a human again.