Albert Barnes Commentary Proverbs 27:22

"Though thou shouldest bray a fool in a mortar with a pestle along with bruised grain, Yet will not his foolishness depart from him." — Proverbs 27:22 (ASV)

Bray - To pound wheat in a mortar with a pestle, in order to free the wheat from its husks and impurities, is to go through a far more elaborate process than threshing. But the folly of the fool is not to be gotten rid of in this way. It sticks to him to the last; all discipline, teaching, experience seem to be wasted on him.