I have never read a Terry Pratchett novel, but he has ensured that no-one will read the ones he left incomplete at his death. The hard-drive of his computer was, according to his wishes, crushed by a steamroller. Story in The Guardian here.
Foucault of course wanted ‘no posthumous publications’, and initially this was strictly followed, then interpreted generously, and is now being entirely contravened (a bit more on that here). Foucault apparently asked Hervé Guibert to destroy the drafts and preparatory materials for the History of Sexuality after his death. He also said various things about destroying drafts himself. It is unclear what, if anything, was actually destroyed, but History of Sexuality volume IV is not the only thing to survive.