There is a way, but doing it also has the nasty side effect of breaking the game.
Kill Malavon before he's has the opportunity to talk to you. Sneak up on him while cloaked, and stab in the back. If he dies instantly from such treatment, the game will go to dialogue mode as you decloak from the backstab, but dialogue mode never ends because he can't say anything, since he's dead. You'd think that since it's only clone-Malavon that it wouldn't interfere with his ability to talk.