Quick answer
Pulverize him and the folly survives. The verse is the book's bleakest statement about the limits of correction.
Read both lines
The image is deliberately excessive. A mortar and pestle reduce grain to flour, which is total: nothing recognizable survives the process. Putting a man in that apparatus is hyperbole, and the hyperbole exists to make the second line unarguable. Even that would not work.
What the verse is saying is that foolishness in this book is not a deficit of information or of pressure. It is a settled orientation, and force does not reach it, which the chapter's own 26:3 neighbor about a rod for the fool's back does not contradict so much as explain. This is a lament rather than a recommendation. The book has spent a great deal of effort on correction and it is honest here about what correction cannot do, and that honesty is what keeps the earlier verses from becoming a method.
Related: Proverbs 26:3 · Proverbs 26:11 · Proverbs 23:9 · Proverbs 17:10
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.